That is a very fair point. There are ecological costs to electronics manufacturing and waste that are not as well understood as lifecycle energy consumption. It is much more complex and appears much harder to solve than energy consumption ... so maybe that's why.
I'm pretty sure you got that backwards ... Distributed systems like Lemmy and PeerTube rely on large amounts of redundancy and duplication. In general, centralized systems are going to be more efficient by default. YouTube is an "ecological nightmare" simply because it's absolutely massive. If PeerTube grows to anywhere near the same scale, you can be sure it will far eclipse total energy usage (and also be harder to measure).