[quote=@Scarifar]
If that's the case, we could also Synchro, right? And if so, then why bother not allowing partners to tribute monsters?
[/quote]

Yes, you could Synchro. It's more for making Fusion work, but nobody here has fusion cards.

And because tributing your friend's cards is rude.

It's also to prevent turn 1 level 5-6 monsters, because at least turn 1 XYZ monsters are bearable.