Why isn't more of Reddit's processing handled on the client side?

Not saying this is a good idea: I bet if it were, it would've been implemented already. But I'm curious as to why it isn't a good idea.

We're doing too many fast_queries (thing.py:fast_query). These are queries on relations between things, like Friends (a relation between an account and an account), Votes, Saved, Hidden, and some others.

The text above, excerpted from a forum comment, put this idea in my head: why aren't friend relationships, a user's votes, etc. cached on the user's machine, so that the database-intensive task of tailoring Reddit's appearance for the individual user might be processed by the client?

It would cut the number of database queries, for one, and hold some user-specific information in a cache that is both free to Reddit, and extremely quick to access from the user's perspective.

This would have the (for Reddit) nice side effect of breaking a few features for Reddit users who use certain ad-disabling plugins, incenting those users to add Reddit as an exception.