As a full-time Youtuber I have a LOT to say about this!
Before I made Youtube my "job", I was uploading every day and taking a full course load at university. When I graduated, I upped my schedule to twice a day. Eventually it felt like too much to handle and I went back to 1. Looking back, I'm often surprised that I was ever able to do 2 a day, but I'll get to how I managed that later.
The truth is, when it comes to doing something you're passionate about, it's tough to be consistent because nobody is consistently passionate. Creative energy ebbs and flows. Trust me, it's much easier to maintain a schedule when you feel good. And most Youtubers who are going full time are riding a nice wave of growth, everything is green, and life has never been better.
Over time, the growth ends and the uncertainty kicks in. Why are my views down several months in a row? Have things changed? Is the quality not good enough? Do I need to switch it up? Does the algorithm not like me? Suddenly the path isn't so clear anymore. You spend more time thinking than doing. You don't rush to make videos in between all the stuff you have to do. Now this is what you have to do, and you'll need to find other things to enjoy in between.
Humans are blessed and cursed with the ability to quickly adapt to our circumstances. Exciting new experiences become normal if they happen every day. Even the joy of attention and money fades over time. It's why the grass is always greener on the other side. You just haven't sat in it yet. In a self-motivated work environment, a source of motivation is pretty important, and it's not always so readily available in the long run.
HOWEVER, there is more to it as well. At least for me. The real reason I was able to upload so frequently in the past is that my videos were... bad. At the very least, they were incredibly surface-level. Hit record, play a game for 30 minutes, barely edit the footage, then put a bad thumbnail on it. Due to many factors, including the ever-increasing demands from the algorithm, I've tried to add a lot more thought, effort and substance to my content over time, but it does mean I'm not able to upload content as frequently.
tl;dr youtube gets less fun over time. still fun though :)
It's interesting that YouTubers who go "full-time" invariably end up releasing fewer videos on a less consistent schedule than when they still were working their day job.