Because they can earn money from it. There is a campaign in particular that has a very high maximum number of posts they will pay you for. It is unreasonable to expect anyone to create 50+ quality posts per week so a lot of members in that signature campaign resort to either spamming generic nonsense or using AI to write their posts. There are also some users participating in campaigns with multiple accounts, which would obviously require a lot of effort if they were creating posts organically, so they use AI to cheat more efficiently.