Of course it is financially advantageous for airbnb to hold $ for as long as possible but in the case of new hosts, I think it has some sense to it. Airbnb does not have agents who go around inspecting new listings. So a new host could be just some 19 year old who is subletting a room in their apartment without the landlord's permission, it could be some totally unacceptable grotty dump, the host could be some sketchy scary weirdo, etc, etc. It makes sense for the guests payment to be held in case the guest arrives to find something like this, in which case the guest would notify airbnb.
Disappointing for new responsible hosts with lovely listings, to be sure, but c'est la vie.
And hosts really shouldn't invest in preparing a new listing if their finances are such that they are going to be severely impacted financially if they don't get booking payments immediately. If you can't really afford to buy new sheets and towels, how's it going to work out when a guest breaks something essential and airbnb refuses to cover the damage because you can't "prove" that the guest did it (a common scenario).