Stopping at some pre-defined point also let’s you offload the mental task of figuring out where to stop, and any possible guilt of stopping “too early”. If you chose a spot to turn around in the middle, it would be someplace that you chose to turn around. Could you have gone further? Was that the best spot? Etc

Reaching the end of the street, it’s more of “oh, well, this is where the street ends anyway, guess this is where I turn around”.