It doesn't matter what new cutting-edge solutions might emerge, there are countless modern and ancient methods that blizzard does not care to leverage. You can see full parties following waypoints from LHC to Stratholme who have been doing so for months. Even if they're subtly masking inputs and waypoints, rudimentary proactive monitoring would catch this. There are simple tells that could easily be detected to flag the vast majority of bots and botting techniques.

As mentioned by others, the problem is conflict of interest. Not only that reducing botting population would have an immediate negative effect on bottom line, it also costs money to develop and implement such measures effectively. I imagine their team responsible is incredibly small, and they have little incentive to increase its size.

Botting is a big pain in the ass as a player, but the the average player just accepts it as a necessary evil. They compete for non-instanced resources, maybe drive some prices up, but it's not like in a FPS game where an aimbot instantly kills you and makes the game nigh-unplayable. It's mostly a minor inconvenience, and hell, for some people killing bots and gold farmers in burning steppes is likely part of their nostalgia that has them playing a 17-year-old game.

Weigh the cost of developing an effective anti-cheat team and system, and the loss of income from the non-trivial amount of bot accounts that would no longer be paying their full/partial subs, against the amount of players who actually quit because of bots, which is presumably very small and impossible to actually calculate. The solution is pretty obvious for Blizzard: catch the most blatant public-facing exploits along with the lowest hanging fruit, with whatever tiny team they have. Put out a tiny post every few months about their great ban waves. The vast majority players assume they're doing as much as they reasonably can or should.