You're confusing google (an advertising company) for unity (a product store).
They're not trying to acquire users for marketing, they're trying to acquire users into their ecosystem and bring more dependence on their tools, their asset store products, and make the unity ecosystem worth more.
It's a land grab. Ditto for VFX industry that they and Epic are trying to get into.
Hopefully it's not the same kind of land grab, where a large company will "nick" successful products off of their asset store and release them "free of charge".
They're not trying to acquire users for marketing, they're trying to acquire users into their ecosystem and bring more dependence on their tools, their asset store products, and make the unity ecosystem worth more.
It's a land grab. Ditto for VFX industry that they and Epic are trying to get into.