Well, maybe it should be. I don't think anyone would begrudge the Godot team taking a small percentage as part of running an asset store, and that means more people who can work on Godot full-time, in addition to the convenience of the asset store itself.
I wouldn't want Godot to go full blown for-profit public corporation, but having more full-timers and more of a real business model like some other open source foundations sounds good to me.
The important thing here is the network effects, not whether you personally can find some store somewhere that technically fits the bill.
Unity's asset store is pretty top-notch and is a good one to compare other asset stores too. Unreal Engine's marketplace pales in comparison.