I have no idea where you get your cars from or what make/model you purchase but with that aside new car purchases with the extended warranty are worth it in most cases. Regardless of how knowledgeable you are about cars/trucks, etc., there are still instances that a warranty is a must have. for example replacing or repair of a transmission (it can be done and I have done it but it is easier a maybe quicker in most instances to have someone else do it), electrical issues, repair of rear axle, bodywork, and engine repair are all reasons to have a warranty.
Yes some people can do the diagnostics and repair but even then certain issues require special tools or need a lift to repair the issue in a timely manner. Also for most people it can save thousands of dollars depending on several factors the least of which are locations where you drive, number of miles per year you drive since increased wear and tear can be a big issue in a few years. Additionally the warranty requires the owners to get proper maintenance (either they do oil/filter changes, transmission fluid, coolant, and checks for tire wear, etc.
In almost all instances, unless you are a mechanic and do this for a living or have more then one car you can use during the repair and/or have plenty of free time to properly do repairs in addition to normal maintenance then a warranrty/extended warranty is a good idea. This is especially true considering the low price of most extended warranties.
Extended warranties don't just extend the standard bumper to bumper, at least in all that I have seen, they usually include a minimum of 3-10+ years of warranty coverage and some cover accidents that cover the difference between what your insurance pays and what they would pay (he manufacturer). Obviously extended warranties aren't for everyone especially those that only have a car for under 5 years since the standard warranty will cover most issues and any recalls.
Extended warranties come in many kinds but in general the ones that are worth it are those that have little to no deductible, covers wear and tear (not just failing parts), and roadside service/breakdown coverage, etc. It can and is definitely worth it depending on what you get and for how long you are keeping the vehicle as well as how many miles you generally put on it. Saying that you would never buy an extended warranty or saying that some issues can readily be repaired by yourself is just being ignorant. Each person has different requirements and extended warranties vary wildly depending on where you buy, from which car manufacturer you buy from, and most importantly who is actually providing the extended warranty.
As far as other extended warranties, most are rip offs and will never be needed or it's cheaper/better to just buy the same product again. In some specific cases it may be worth it but in general they aren't. Every person needs something different but aside from car extended warranties (from a reputable company) most others are just crap. It's just ignorant to group everyone in the same boat and then say that everybody should do what you do.