And make money how? Creators are dependent on platforms to get and monetize attention. People aren't going to stop using Instagram and YouTube because a few superstars only post content on TikTok.
TikTok has a fund to pay people to make content to get them up off the ground.
Insta, despite it's growth, has a decay problem in that it's out of the zeitgeist of the key demos. it's growing in developing markets and maybe might have some incidental sign ups, but people are not flocking to it.
And TikTok - depsite it's popularity, has a very 'fad' feel to it. While FB has a bit of the 'e-mail' incumbency in that it's how people communicate with family and user groups, TikTok is a bit more fleeting.
These platforms would get hurt badly without a lot of those middle and upper tier content makers.
The current situation is a rational entente actually: Insta makes ads, influencers can make a buck however. The more popular YouTubers will monetize with ads anyhow.
The fact that you call them "Youtubers", identifying them using the trademark of one particular platform rather than a generic descriptor like "video content creator", suggests that there is not quite as much healthy competition as you claim. Most video companies other than youtube only compete with youtube in a narrow sense; a great deal of the content on youtube does not fit on tiktok's platform, which is only good for short-form content. Netflix only hosts movies or TV shows. Twitch is for livestreaming; other kinds of content don't fit into the paradigm of twitch. Vimeo has never been a good place for off-the-cuff home movies, they too try to compete with youtube only in a narrow domain, not across the board. The few generalist video hosting companies other than youtube are all jokes that are faaar behind youtube in viewer counts.
If the platforms wanted to come after the money, they could. I suspect they're still in the growing phase. But I imagine in the near future, they'll start demanding all ads go through their platforms.