At first, they had few ads, or ads that were always skippable
Now that they have a near monopoly, they're adding more ads, and making more of them unskippable.
To watch a lifesaving video, that was posted at a time when there were no real ads or all ads were skippable, you may have to watch a series of 30 second ads.
Then there's the fact that virtually every video requires ads before you view it, when you might only need to see 10s of the video to know that it's not relevant. Putting multiple ads before you're allowed to see those 10 seconds is an asshole move. Then there's the sheer quantity of ads. The last time I tried to watch a few music videos without an ad blocker I think it was at least 25% ad time.
Removing the ability to skip is also a major asshole move. The whole justification for skippable ads was that consumers wouldn't skip ads that were good and relevant. If the advertiser was doing their job and making good ads, and YouTube was doing a good job and finding the right audience for those ads, then in theory the ads wouldn't be skipped. If an advertiser was upset that users were skipping ads, YouTube could push back and say that either their ad sucked, or that they had mistargeted the ad. Now they seem to be admitting that their ad targeting is bullshit, or that they don't care if the ad isn't relevant to the user, the user has to watch it regardless.
But, most of this hinges on the enshittification happening once the platform has become a monopoly. If YouTube only had a 20% market share, asshole moves would push people to competitors. But, the key thing is that they lost money or barely broke even until all their competitors had folded, and then they started making things worse.
/r/assholedesign refers to user hostile design. Ads are generally unwanted and def unwanted if the person has an ad blocker. Putting a nag pop up is inherently user hostile.
Ads on YouTube--like everywhere else on the web--became so obtrusive that it's nearly impossible to view anything while still having all those ads. They're making their content unwatchable for anyone who can't pony up for a subscription.