That's actually a great question. As a PPL user myself, I think I can shed some light on it.
First of all, we're not saying "oh yeah, we get to do arms 2 out of 3 days". We're thinking we need to have one day for chest and triceps (pushing exercises), and then another day for upper back and biceps (pulling exercises).
If we do all that in one day, it's gonna be a LONG day. Also, the elbow joint can very easily end up with tendonitis if you do all of that pushing and pulling in one day.
The knee joint isn't as susceptible to tendonitis, so we can do all those extension AND curling exercises in one day, and not have to worry so much about that.
However, on leg days, we do the heaviest lifts of all, namely the squats and the deadlifts. It's usually recommended to limit these a bit because while your muscles may handle the pressure, all that weight can really take its toll on your lower back, not to mention the central nervous system (CNS).
Of course, this is something you can tweak as you get some more experience, and learn how much your own body can take. If you find you'd rather consolidate Push and Pull into one single upper body day, feel free to. Also, if you'd rather do a PPLL split, that's fine too!