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The game has ads on Mobile, that's why it is free. Also, it doesn't have any cosmetics, apart from some hats.

The game on PC is very cheap, it has a discount right now, and you get a bunch of cosmetics when you buy it.
You cannot put ads on a PC game, it doesn't work the same way.

Devs need to eat, so they need the money. That's how it works.

You should consider getting it on EGS then, It's way more cheaper on the epic games store. Less than a dollar i think.