video streaming on mobile data is everywhere and ISPs responded by fattening up their networks with newer, better, faster tech, like 4g/5g

Yeah, but streaming from your phone to a streaming service, or whatever, hands over the job of distribution to the streaming service.

Streaming may be 'everywhere' but how many phones are streaming at any given moment? 0.01%? It's probably not even that many. Now how many are watching TikTok? How much more bandwidth would they need if the TikTok client was also serving videos to other TikTok clients?

Now, could you obfuscate the video with encryption, etc. to make it nearly impossible for cell phone companies to stop it? Probably. But, you'd need the cooperation of the Google Play & Apple stores to make that happen (on non-rooted devices), and it seems likely they would take the side of their cell provider partners.