Why bother making a “main product” at all

My guess is legally protecting themselves against potential new laws and regulations, so they have more time to make changes or continue making money if these practices get outlawed.

Why is this exclusive to the mobile market?

It’s not; it exists in computer and console gaming too, just to a lesser extent. As to why, my guess here is that mobile has a lower barrier to entry, and they’re always accessible where a computer or console might not be. There’s probably some sort of “cultural” aspect here too, where it’s already prevalent in mobile so people are more accepting of it, but computer/console gamers had their own culture where this didn’t exist for many decades and so won’t be as accepting.