V1 is based on the full (or planned) length of the runway. If some twit taxies out in front of you, then that figure is not worth anything.
If you can't stop in time then it may well be worth getting the plane into the air at a speed at which it will physically fly. It'd be better to do that than to run into another plane when one or both have a lot of fuel on board.
That's why they pay us to make decisions like that.
I know what the plane can really do, so I am better equipped to make the decision whether to try to stop or try to go, rather than make a seat-of-the-pants one based on sheer guesswork.