You're right, you can watch videos on your phone. But you're also paying for it. Every month. My cell phone bill is $120 / mo for fast, "unlimited" data (really 22GB at 4G speeds) per month.

But while the car's internet is free for you, it is not free for Tesla. This is true for the hundreds of thousands of cars they have on the road. It is simply not economical for them to stream GBs of data for every car forever. Years after they will have stopped making money from your car's sale.

Doing it for specific short clips (like when sentry is activated) is much more sensible, and likely the limit of what they'll do.

I work in building large scale, distributed systems. It's not as feasible, simple, or economical as you seem to suggest. Especially when the client connection is an expensive cellular one.