Yeah, not sure about this either. Logic?
Leveling comes from time in game and multipliers (and gen bonus). Time in game is the same regardless, and multipliers come to those who win rounds (which honestly depending on the mode, 1h/shield, cav or other classes are the ones who win those, not necessarily 2h), and as for valour - shielders are the best way to farm points for valour...
Leveling in eu5 is easier when you're a melee build. And the easiest when it's a 2handers.
Cavs can only mount "decent" 3 riding horses if they're nearly naked on top of it.
Range always have their best (x)bows and (bolts/)arrows mw, and since the lack of diversity of (x)bow, they have to use shitty ones, with shitty ammo, or just go melee with 0 PS.
Throwing is okay. But the lack of number of throwers, means a lack of ammo. And 4 stacks of throwing weapons is hella expensive.
So melee.
1handers need to block the first swing of a 2h. Unless they use a shield. Wich would mean a slow shield.
Polearms need the agility to effectively spam their weapon, so it's also less effective.
This is on eu5 only, of course.