Why does a game take so long to load? What is it loading? / Suggested improvement

Apologies for posting this here but it won't let me post in "Client Discussion".

Compared to Dota, there is a rather long loading period to start the game. In Dota, when everyone has chosen their heroes, the game shows a splash screen for maybe 1 second, and then the game is up and everyone is running around.

Obviously things have to be loaded before the game starts. But why not do all this loading earlier? If LoL is loading things client-side, this can start even from when the player initiates matchmaking, which normally lasts about 3 minutes for me (that's a lot of time you can be spending doing useful loading). If it's information being transferred over the network between players and server, this can begin in the background when everyone is in the lobby, choosing their heroes.