So why does this happen?
IMHO the answer is: "they control the distribution channel".
That is precisely the same mechanism the entertainment companies had used for years in the era of vinyl, cassette, and VHS.
YouTube has reached that status through at least three intertwined paths that I can think of:
1. Sheer scale makes it the default destination for video content, both for producers and consumers.
2. Sheer scale and the non critical nature of its data also means that their storage costs are are much lower (per unit of storage) than what any other new competitor could afford.
3. Sheer scale and profitability means they can sustain a very decent level of performance that no other platform can currently match (at scale).
So, again IMO, their dominance won't be successfully challenged until a technological change occurs that will destroy their distribution model. I don't know what that could be, but it could involve, for instance, a ×1000 reduction of storage / transmission costs (e.g., hyper efficient local AI hardware that can generate the video on your computer, quickly and cheaply, based on a textual description of the content?)
What I do not know is why Amazon or Microsoft have not decided to compete on that space.
Lastly, note that the above is only applicable in the West. Does not apply to nations that are not technologically dependent on the US at the consumer level.