Twitter has a perverse incentive to allow them because it is directly related to their stock price
Compare that to Company B that has 500k DAU and flat. Not so good.
Now what happens if it turns out that Company A’s DAUs we’re actually bots? Suddenly, the audience isn’t worth anything, because bots aren’t an advertiser friendly audience. So now the stock should at least drop in accordance to the real-DAU, and maybe even less to price in lack of confidence in the future.
The last big round of fake bot purges saw significant percentages of users removed from prominent accounts like Obama’s.
If normal people don’t know it’s a problem - why would Twitter want to fix it? Add that to a real financial hit if they do. Not “can’t”, “won’t”.