The right to abortion is a downstream right of bodily autonomy, granted by the constitution under the word "liberty". It's right there in the preamble, not even an amendment. You don't have to get deep into the document to find it.
You have a right, as a citizen, to do what you want with your body, so long as it doesn't violate the rights of another citizen. The unborn are not citizens. They're not even people, as they have no long term memory capability or ability of rational thought.
It's pretty clear. It's the same reason you can't be forced to donate a kidney even though you have two, even to save the life of the President.
I guess I'm just real confused as to why you think it's not covered by the constitution. Do you really believe the only rights granted by the constitution are those specifically named? Because, oh buddy, you'd have a real bad time if that were truly the case.