Because of $$$ and all the shady stuff that happens behind the scenes at Blizzard.
They let it get out of control intentionally and they dont ban hackers or bots because the hack and bot accounts just generate more money each month for Blizzard.
Actually I believe that classic’s total playerbae isnt as big as it seems as well. Most people are running multiple accounts to cheese professions and to do other cheating nonsense like multiboxing or running botting programs with they’re alt accounts. Id say roughly 30% of classics total population is just active alt accounts that people have to cheat the game with.