Most of the above arguments are very good arguments - from the host's point of view, but airbnb might feel that annoying a few new hosts and filtering out some of them the hard way, is no big deal to them, compared to the unknown benefit they expect from the experience.
I'll try another argument: I use IB on one of my listings for 9 months now, with no problem so far. It makes bookings easier, when I'm out of Internet connexion for a few hours. If I'm online, it makes no difference. I accept most people anyway, and the setting "recommended guests only" forbids new members to use instant book. That allows me to clarify a few points, that a new guest might not have realised reading a clear description or looking at 20 pictures.
But I can use IB on one listing only, for logical reasons: I rent either the whole apartment or the guest room with a single bed or my bedroom with a double bed. As soon as one of them is booked, the other listings are blocked.
If I rent the whole apartment, I go to my other home and this one is blocked too. But the system does not allow to connect them in a wat that one booking blocks 3 or four listings. I have to do that manually.
If IB was forced for all listings, I could only list one of them, would earn less money and airbnb would earn less service fee too.
New hosts, who may have no way to predict which offer will work best, may experiment at the beginning with offering the whole place or rooms or both. With forced IB, they could only set up one kind of listing per apartment, either full place or room(s). Probably only one room listing - even if you have 2 or more guest rooms, the idea to have different sets of strangers dropping in on you without your say so would be very frightening. That thought would even give me pause after 12 years of hosting and grewing up in a family pension 😉
IB book is a perfect tool for more commercial set ups. Additionally, if the main motivation in renting rooms or flats is the money, declining people for racist motivations would be bad business.
The question how to avoid discrimination could be much easier anwsered, if airbnb allowed a clear distinction between private homes and places solely destined for seasonal lease.
There are guests for both and most times they don't feel very comfortable if they get the other thing.
Some guests use the IB filter to get a cheap or fast hotel room booking. I had a few of those. I don't believe that they are mean or breaking rules intentionally, searching for a new host on IB. No, they search a room where they don't have to give a thought or pick up a dropped item. They are bothered by hosts setting rules and a living in host is bothered by their behaviour.
A filter Private Home / Neutral place would avoid that, and people who fear discrimination could use it to search for commercial settings with lower probability to turn away anyone.
IB can be an additional option, a tool to speed up the process, nothing more.
It's a bad idea to use such a tool to solve another question, especially if you are too afraid to name the problem even.
