It's possible the manufacturers mandate that the DMEs do this. After all, if they want to be an authorized dealer, the manufacturers often do require a lot of things.
I don't know which, if any, manufacturers DO require this of their dealers.
That would be nice. I hope they do require it. It seems you should get something for that substantial markup over online costs. And to be fair, it seems like many people do - at least in the mask fitting area. I didn't but I've long since parted ways with that DME and indeed it has long since ceased to exist having been gobbled up by one of the big chains.