One reason: It helps makes the bot accounts look more legit. It's why Facebook bot accounts like random legit pages.

This was my thought too. It's easy to pretend to click one ad in order to make that ad money. But it's also easy for Google to tell apart a huge number of accounts that all clicked that one ad and no others. However, if a huge number of accounts simply click a whole load of ads there's basically no way to tell that it was targeted at any particular ad + they can bot for multiple ads at once.