Some accounts also get stolen with phishing methods or used with fraudulent credit cards, that incur chargeback merchant fees for Blizzard. The “paid” amount plus penalty costing Blizzard a lot more not to mention the damage the botters do ingame.
For example in video 2 I think they explain how botters make very cheap accounts not profitable for Blizzard at all in here (thanks to that poster btw for linking):
For example in another F2P game Valorant & League this is explained in more detail why it is so difficult:
An ex-dev that worked at Blizz:
If you have any good ideas don’t forget to submit them through the ingame suggestion box!