Because of voicelines, hitbox/hurtbox, geometry, color...stuff like that. Cosmetics aren't allowed in any high level esports game except csgo I think, which is like knife skins and stuff like that, not character models
Multiple reasons teams practice against other teams with default skins because 1 everyone has them and 2 it's an easy Base to learn
this means teams are more used to seeing and aiming at certain skins
2 some leagues make you play on new accounts
3 pro players will analyse playstyles so say 2 people on the team play dva and they play her completely differently if the two people swap dva it would throw off the enemy because the playstyle is different
If they had different skins, this wouldn't work because the enemy would notice the change quicker
I think is that way so people that is new to the game or is their first time seeing it they will recognize the heroes easily and keep track of whats happening
Because you dont log in to your account if you play the world cup. You play a different version of the game, like the ones they put as demo's on conventions. So it's all basic game stuff
1. It's a new account so they can't unlock skins.
2. Even if they did, they most likely use the option 'Disable Skin' for 'advantages' for skins. (I know there is still a red line but I've seen other games do things simalar.)
They use fresh accounts and also they would want to make sure everyone is on a completely level playing field. Some skins have better colors for certain maps, some have flash objects, etc.