It's just incentives in general. Humans are self-motivated creatures, and in the absence of meaningful consequences they'll often do what they're incentivized to do, even if it harms others.

The internet is uniquely devoid of consequences (esp. reputational consequences, social faux pas, etc.) and makes effort expenditure minimal. So you get lots of bad behavior.

I think "ads vs not ads" is maybe the wrong way to model it. Ultimately people are just doing what benefits themselves across every dimension possible.