This large pipe coming out of the roof

It's a cleanout for the sewer line. You can shorten it to grade level, but you cannot remove or pave over it.

You'll probably never need it, but if you do, you'll want quick access to it.

Find a better store, if the ones you are going to don't have 2" PVC in full sticks (or possibly look higher up in the store you are in, and get someone to drag it down.)

In MY house, the only place you'll find 1-1/2 or 1-1/4" is the tailpieces on the sinks - as soon as it hits pipe, it goes up to 2", because it's not that much more expensive and it greatly reduces the odds of having any issue with it. I don't recall having any trouble finding it in full sticks. The main line is 4" (not 3") for the same reason. Builders looking to squeeze maximum profit don't do that sort of thing, but builders who plan to live in the building for a while sometimes do.

In retrofitting, you might need to use 1-1/2" due to space constraints. In any case, if you are redoing the pipes, you should probably bring the venting up to current code while you are at it.