I don’t know and this is extremely uninformed conjecture

I think it comes down mainly to two things:

Dedicated TV stations use purpose-built infrastructure that’s designed specifically for broadcasting a small number of video feeds to a large number of consumers. The internet is a generic system that isn’t built for this in particular, so using it for this purpose creates a large amount of overhead that isn’t needed for normal broadcasts. This results in various effects other than wasted energy, such as internet-based broadcasts having much more delay than dedicated TV broadcasts. But for this discussion, the main effect we care about is that a lot of communication is wasted on “keep alive”, retransmission of lost packets and other such nonsense that wouldn’t exist for dedicated broadcasts.

On the other hand, the internet is already there, and requires no new/extra infrastructure. It could be that the environmental cost of building and maintaining a parallel infrastructure just for TV is greater than the cost of just using the internet that’s already there.

There’s a caveat for both sides:

With dedicated broadcasting, it could be that the energy required for broadcasting a powerful signal that reaches all the customers is higher than the energy required to reach the same number of customers via the internet. You’d have to run the numbers, which would require a lot of research to find.

But it also could be that online streaming creates so much congestion on the internet that it causes demand for more/better/faster internet infrastructure than wouldn’t otherwise be needed, causing additional waste that offsets the benefit of using infrastructure that “already existed”. I’d wager this is provably true, that online streaming is the leading cause of more internet infrastructure - but like the rest of this reply, I did absolutely no research to prove it.