You’re obviously not involved in the content creation world.
My partners main income source is selling a product via Instagram. While she puts a lot of effort into building her brand on other platforms to diversify as she recognises the risk, once you have an established and profitable audience on one platform you’d be silly not to continue to put effort into growing that. Also what works on one platform is different to the others, it’s not a simple copy and paste.
People generally are aware of this and try to diversify, but it’s not as easy as you might think.
Posts that get 5m+ views on one platform may only get 5000 on another.