Same reason they did not block us yet - better to have some users that block ads, than have those users on other plattforms. Now too many people used ad blockers - so step by step they want to "encourage" people to look at ads again or pay for premium. But too strictly enforcing it from day one - would probably mean too many people going away. But giving more time and nagging, more might enlist for premium.

(I won't, I seldom use youtube)