Copyright, you have to provide all the copyrighted material yourself so that they can't be sued for distributing it.

Man, what a waste of resources. It'd be funny if the client side just did a hash of the "uploaded" files, told the server the hash, and then the server can compare the hash and use the server copy of the assets, to save bandwidth. But as "What colour are your bits" (1) say, that'd still probably not be legal.

Oops, it's a browser based game, you still need the assets on the client side, i.e. in your local memory... never mind

(1) https://ansuz.sooke.bc.ca/entry/23