They do what they can but it's not a battle against botting, it's a war. Accounts can be made for free with nothing more than a unique email (which can also be made free), which allows botters to create them in massive amounts. They will always get some games in before they are caught, which sucks, but there's not much that can be done before they're discovered. Bot bans are done in waves so that the botters have a difficult time figuring out how they get caught; however, they improve their botting methods too and it's an ever shifting goalpost as Riot gets ahead of them again.
It definitely does suck, but Riot is not blind to the problem and they don't stop banning them or improving their detection. Try to bear with it in the lowers levels which harbors most of them (they typically get banned before getting far) and don't forget to report them in the post-game. If you find level 30 bots please submit a Support Ticket to help Riot catch them.