It looks like that's a pretty recent development, at least within the last 30 days:
YouTube TV gaining full DVR playback for Disney, Fox, NBC, and more
It's good for YoutubeTV subscribers that this functionality changed for now, however, I'm skeptical this will continue to be the case long-term.
Alphabet, Inc's main source of revenue is advertising. Pare that up with how the video streams are sourced in the first place -- Hulu gave a talk at an AWS conference last year highlighting their content flow (which translates pretty readily to how pretty much every service is doing this):
The key takeaways are that the origin streams come directly from the content owners, and are re-packaged by the streaming TV providers to deliver to any given service's clients. Within the metadata of the origin streams they are preserving the
SCTE-35 coding to signal ad insertion points, as it common in other digital streams today such as satellite feeds where the DVRs are doing local ad insertion. What's particularly interesting in this talk, is they point out the ownership of Hulu being content producers, and thus the heavy influence to have the mechanisms in place for non-skippable ads.
When you combine a company that primarily makes its money from ads with video streams that are already nicely encoded for ad insertion points, I find it really hard to believe that the long term play for YoutubeTV is perpetually to be a $35/40 service where you can watch all the TV you want and skip all the ads at will.
Streaming being a winner based on price is a short term game. I don't say that as a way of saying "streaming is doomed" or anything like that. There are still numerous advantages when it comes to providing enhanced video quality (4k is much more prevalent in streaming), mobility across multiple devices, and having substantially more levers to customize your content selection and discovery. I get my value out of Hulu just by being able to side-step DirecTV's horribly clunky VOD / mobile app platform.
If I have a beef with streaming, it's around the constant advertising with the taglines of "Stop paying too much for TV" or other Bullsh!t like that. It makes it sound like you'd be a fool to not cut the cord and switch to a streaming service, but the reality is it's a bit of a minefield out there.
No contracts! Switch all you want!
Oh, you want to watch AMC? Hulu's off your list.
Oh, you want to watch A&E networks? Scratch Vue and YoutubeTV off your list
You want to watch Viacom channels? Enjoy 30fps video on Sling or Philo, or "enjoy" the dumpster fire that is DirecTV NOW
You can't stand 30fps? Skip Philo and SlingTV
You have an Amazon Fire TV device? No YoutubeTV for you.
Oh, and in each case, you have to walk away from all your DVR'd content when you switch services.
Matching what networks you watch to services has become complicated enough that now sites exist just to help people with that very problem:
Suppose... you could design your perfect TV service