There's nothing wrong with asking why something happens.
So, If we were in a thread about new medical procedures that affect HIV transmission, and somebody asked why the simpler, common sense, Cabbage Patch theory of Child Origins was being ignored in favor of the S.E.X thory those silly scientists propose, there would be no reason to be dismissive? If I thought somebody asking that sort of question actually meant it, I'd try to give them an honest answer*, but why shouldn't I assume they are not really honestly confused, but tossing in a deliberately spurious question, in an attempt to throw the argument off track, politicise it, ot just plain troll? Sometimes, you read a question, and think, "What are the odds the person really doesn't know THAT and is really honestly asking to become more informed?"
There's some "simple, common sense" reasons to doubt that sea levels will or should rise uniformly, and most of us learned the first one of them about 3rd grade (in the US system).
1. The oceans aren't starting from static equilibrium - if they were, there would be no currents, as all the water would have already gotten to where it was going. So the question assumes something we already know is false, that the oceans can swiftly get to a static equilibriums state. Knowing that there are currents is enough to make a reasonable person doubt the question, Water keeps rushing from place to place all over the oceans, it never stops flowing as a whole, and it has from times well before the contemporary (AGW related) era, so why does it seem reasonable to assume that NOW it should all swiftly get to the lowest spot possible and stay there? How how old were you when you first heard about ocean currents?
2. Oceans are very large. Why does it seem like common sense to some that changes happen near instantaniously in such big objects? Wouldn't it be more common sense to find out something about the time scales for other changes in the oceans? How old were you when you learned there were tides, and did you learn that high tides are higher in some places than others, and at some times of the year than others? That's probably something people who live well away from seacoasts start getting exposed to by 6th grade or so, but if they missed it then, there's typically this course in junior high school, usually called something like Ecology or Earth Science. It's the course people who want an easy pass on their required science credits take, if Introductory Chemistry or Physics seems daunting. (all this assumes the child lives in a state with at least some science requirements for secondary education, but despite the problems of the US educational system, the vast majority of states do have science requirements) .And the majority of people live in cities, which are very frequently on seacoasts, so many people pick up many more facts about tides very early in life. Now how do I give a person a respectful answer, if that answer implies they went to a vastly substandard school system, or failed a 'bonehead' course, or ignored something they were near-constantly exposed to in their formative years? If I give a deliberately dismissive answer, I'm not honoring the principles of free, scientific enquiry, but if I ask the questions needed to find out what the other person doesn't know, I'll probably end up insulting the person anyway, and if it's deliberate trolling/politics, the person will jump on any answer and spin it in the worst possible light.
3. The Earth is a flattened sphere with some odd buldges, not either a true sphere or an egg shape. We're not just talking mountains and valleys here, but larger scale differences, caused in part by the Earth's rotation, and by the continents themselves. Many people don't pick that fact up until high school or even college, but it was probably offered too in those same Earth Science type classes. Is that enough to explain why everywhere doesn't see the exact same sea level rise? If I didn't know one way or the other, I would at least consider the possibility. Maybe the Earth's rotation means that sea levels will rise more near the equator, and less at the poles, or something like that, or maybe water piles up a bit towards the leading edges of the continents (leading being in the direction the Earth rotates). Without knowing why the Marshall islands levels are rising faster than some other places (what other places), wouldn't it make sense to find out what other places they are being compared with? (In fact, both those effects happen and have an impact on how much sea level rise some places see - there's an actual answer that applies to the Marshalls).
4. 'i kan reed (749298)' has given a fair explanation for part of this question in the same thread. I just want to point out that many people learn at an early age that the Panama canal uses locks to raise and lower ships passing through, and that these are absolutely necessary because the sea levels on the two ends are different. Again, that's the kind of thing that might make the questioner realize that the whole real world isn't staying in line with the base assumptions of the question - it's not just since AGW hit the news, but all sorts of "ocean facts" that seem to say there's something the questioner doesn't understand, but has assumed they already do. Now that bit about the Panama canal is a bit esoteric. If a reader lives in Europe or Asia, they might well have completed a good education and never run across it, and they may well have heard much more about the Suez canal, which seems to work differently re. sea levels. I'm also sure many US schools rush through Panama Canal - Teddy Roosevelt - Yellow Fever - there we're done with that decade. But how do I know if that's an informative example or the poster will come back with some B.S. question about what relevance does a man made canal have to do with natural phenomina?
*The temptation to say I'd try to give them a straight answer finally overwhelmed me. Bite me pun haters!