Splitting it by default would be weird, since a lot of people
don't want it split. When I go out to eat with my family, we don't want four separate checks, and most couples going out don't want two checks. I don't even want separate checks with friends I go out with regularly, because we just alternate approximately instead of trying to calculate everything accurately to the nearest dollar.
In my experience the main groups who do want splits are college students and coworkers, but I'm not sure what proportion of total restaurant meals those two groups account for. Anecdotally, restaurants near colleges or in college towns are much more likely to ask you if you want a split check, though.
As for logistics, I think the main one is that it requires an extra level of keeping track, especially for larger groups. Instead of just punching in "two beers and a margarita for table 15", the waiter then has to enter in who ordered what, either while punching it in initially (if the system supports it) or afterwards. It's considerably more complicated to track 30 items ordered by 6 separate people than to just track 30 items ordered by a table. And then the waiter additionally has to ask for details on what to do with ambiguous things, like appetizers. People get pretty absurd with that too: they want one appetizer split between 5 of the 6 people (because the 6th person didn't eat any of it), another appetizer split evenly, the pitcher of beer split between a different 4-out-of-6 people. Oh, and they want two of those people on the same check because they're husband-and-wife, while the other four get 1-person checks. Etc., etc.