>>55823865In-universe reason? Because Umbreons evolve out of really liking their trainer and would be protective of them. Would make no sense to have a Pokemon evolve by hating you, yet then have no problem letting you send it into fights and command it.
Game mechanics reason? Because Pokemon start out with low friendship anyway and intentionally lowering it would be a bitch since almost anything you do increases it, about the only things that decrease it are letting it faint or using herbal medicine..... also Espeon and Umbreon debuted in Gen 2, which also debuted a time-of-day mechanic, and they wanted to use them as an example of that mechanic.
There is a move that is stronger the lower your friendship is (Frustration), but it's pretty pointless for both the above reason and because it has a very similar sister move (Return) that is more powerful the higher the friendship stat. Also just about every Pokemon that can learn Frustration (with the exception of one in a very special case) can also learn Return.... along with an additional two that are also special cases that can only learn Return and not Frustration.
