When is something going to be done about these bots?

I'd be more inclined to believe that if not for the constant march of the bots. Whatever they're doing clearly isn't working.

Or the people who farm accounts with these bots don't care that they get banned because they can sell the accounts before that. As long as people are stupid enough (or simply willing) to buy botted accounts that just end up being banned, this will keep happening regardless of how good Riot is at detecting them.

And it's not like detecting that a player who never gets a single kill, always picks jungle, always gets executed by the jungle minions, never says a word, etc, is some herculean task of machine learning and math.

Of course it isn't. Further evidence that these specific kinds of bots are not trying to outsmart the system to begin with and simply aim to get to level 30 and be sold before they are banned. But just because these bots aren't trying doesn't mean nobody is. And instantly banning bots when they are detected gives people who are trying to create scripts and bots a lot of information as to how Riot does detect them and would allow them to adapt even quicker than they do already.

I think you underestimate how much money people get for botting accounts, and how easy it is to do.

It's annoying, but it's a futile fight for Riot.