Why is Riot refusing to rebalance some champions?

As I've become an ARAM-only player and have over 6000+ ARAMs, there are some champions that always perform well and some that always perform horribly and are just not worth picking up.

For example, no matter who's playing him, Graves always ends up one/two-shotting everyone. The same could go for Akshan, for example.

Why is RIOT no trying to balance things out? Because it's been like this for months (maybe even years).