For me, it's because the finish line is actually a starting line for another thing altogether. The starting line of a race that I don't really want to run. It's like all this time you've been building, you've just been in a car on your way to the race, then you got there and are in the parking lot and can see the finish line, you just have to get up and walk over to the track. But, the realization of you not actually being prepared for the race sets in, did you bring your shoes? Have you been training? Do you even enjoy running? Meh, let's go get ice cream instead.