In the case of YouTubers, if you already have thousands of followers, video gear, lighting, hard-earned performance skills, and video editing proficiency, then it is entirely rational to try to continue to leverage those investments for your future income.
I've got two decades of experience in software engineering. It's not a sunk cost fallacy for me to continue looking for software engineering jobs, it's me trying to capitalize on that investment.
The real hard part is figuring out when some past investment like that is winding down such that you're better off discarding your potential future earning from it order to spend that time building up a new investment. You don't wait until the copper mine is completely empty to shut it down. You close it when the copper you can extract is worth less than you could get from spending that time elsewhere. Making that calculation for our intangible skill investments is really difficult.
1. They record a video of something on their phones and upload it to share with their friends. Practically every human with a smartphone does this at some point.
2. Maybe they enjoyed that and the feedback they got from their friends, so they make it public and maybe upload a few more. They start thinking about sharing videos to an audience bigger than the people they personally know. This is a completely natural transition because most people on Earth are frequently watching YouTube videos that were shared just like that.
3. Now it starts to feel like the video itself is the goal and not the experience they were having that they happened to film. Instead of "we were doing X and recorded it" it's "I'm making a video of X". They make more little videos and upload them. They enjoy seeing their production skills improve, and it feels good to make things. Their friends think they're funny and it feels really nice getting a positive comment or thumbs up on the video. They are getting zero money. It's entirely about intrinsic reward.
4. Some small fraction of the people who reach 3 stick with it enough and have the right personality/video skills/topic/whatever to get a bit of traction. Now they're making a tiny amount of money and being a "YouTuber" starts to feel like more of who they are. They start to wonder how far they can take this.
5. Now that some trickle of money is coming it, they naturally start thinking about investing in what they do. It makes sense to maybe get a better camera, or buy Final Cut Pro. They start putting real time into this, at the expense of other hobbies and activities.
6. At this point, being a YouTuber starts to involve actual trade-offs. They're putting enough time and money into it that they are sacrificing other opportunities. Raising their production value and keeping up with posting frequency means they need to make videos even when they don't feel like it. They start doing the calculation of whether it's worth it.
7. Those who decide it is worth it now need to start taking it more seriously. The sacrifices they are making are clear and fixed, but the rewards (popularity and money) are less predictable. Now they are at the point you mention where they start really caring about optimizing for YouTube's opaque algorithm. Those who do it well may actually be able to make a living, but it always feels tenuous and uncertain. Staking your income on the combination of popularity contest/machine learning algorithm/ad platform is pretty sketchy. But for some it actually works. And, at this point, you're in pretty far, already have a lot of experience and time put in, so it makes sense to see how far you can ride it.