I think it is because it is also harmful to be at 100%, not only to charge there. So a docked deck will be longer in a harmful state than a mobile one. In the case of a mobile deck there is a clear use case for charging to 100%: Longer battery life.
But you are also correct: If you’re not using the full battery charge during everyday operations, you should also enable that and also buy one of those 4-5m USB-C charging cables
If your deck is docked, it will always be at 100% charge
Because you can now just set the charging level to 80%, plugin your Deck into your dock and it will stop charging at 80% and then just use passthrough power.
How would something like that even work? Create a driver that phones home is somehow able to figure out the location of the computer by some method that is not easily defeated with a VPN? Even a GPS chip on the cards won’t work in a datacenter rack
Batteries really do not like to be at 100% charge. So if you use your SteamDeck mostly docked, it will be always at 100% charge and this prevents this. It’s a nice, useful feature.
See here how Norway is doing it: https://www.theguardian.com/environment/2023/nov/23/norway-heat-pumps-cold-heating