Authentication and Authorization of users is one of those things I don't like storing data for. In short it's bad for the same reason storing credit card numbers (or any PII) is. Not only is it not my app's core job, it's a disaster waiting to happen in a breach. User provisioning is also one of those things I hate having to do when everyone on the web already has an email identity.
So the question remains: why is it that your only real options for OIDC just happen to be the big boys? One of the major goals for my CMS is to disintermediate from the big aggregators, so this is a problem I'd love to have solved. Why can't my shared hosting account be my OIDC provider? Furthermore, why do I need 9 different logins for services on the same server? It turns out there are some technical and organizational hurdles that have to be overcome.
The primary difficulty is that there is no standard for how you ought to advertise the available JSON Web Tokens to pages. It's standard practice to use Double Submit cookies and synchronizers in DOM components to prevent XSS/CSRF, rather than stuffing JWTs into LocalStorage. This is why every OIDC process does the two-step over to another domain, as the cookies are SameSite and can't be used anywhere else. The practical consequence of the status quo is that every single oauth implementation is a hardcode rather than iterating over your active and available logins.
I suspect the big boys are content with this status quo and would resist any solution that didn't allow them to stay at the top of the list of available providers. Existing objections have to do with the user privacy (should I advertise my active CrazyFetish.yikes login?) provided by the status quo. To keep such control in the hands of the user, any such feature should be opt-in per service. To make that happen, browser integrations would have to happen.
Therein lies the rub. Incentives don't align here, as the big boys are also the browser vendors. Such a regime that preserves privacy would necessarily not preserve their ability to get free advertising on every login screen worldwide.
This could be made less objectionable by throwing some bones to both the tech firms and developers. Both would like to display a branding image & copy for these auth providers alongside the login prompt. Thankfully, this could easily be stuffed into LocalStorage with a standard name without risk.
It would also not be the first time that browsers have had a pre-baked whitelist of services for self-serving reasons. To be entirely frank, I'd prefer that they do such a thing in this case rather than force every app developer on earth to write extra code for no good reason. I'd rather be happy than right and especially in this case. I have a feeling you would too.
The first problem is that they went about discoverability wrong. They use the .well_known mechanism in vhosts. The more standard way would be something like a TXT or SRV record on an appropriate subdomain. LetsEncrypt found out the hard way that vhosts are a lot easier to compromise than A records. It also doesn't hurt that DNS is basically the LCD of web features. Almost everything has to know how to talk DNS, not everything talks HTTP.
From there it's essentially a matter of integrating SASL into applications, as it already supports Oauth tokens. Given most of the backbone services out there aren't hostages of large tech firms like browsers, the server end shouldn't be hard to gain momentum. The trouble will be with OS vendors, as they'll have the same problem as browser vendors. This pattern repetition suggests that the real problem is that we actually treat identity and authentication as a tack-on component rather than as a first class service.
This suggests that gaining the requisite momentum to solve this problem can't really be achieved externally. The OS and Browser vendors are going to have to want to solve this problem. Until then, developers will have to write auth code they don't have to, and put up with services written by people who don't understand security concerns. Which is the biggest selling point to me -- The OS and browser vendors could do humanity a favor and extinguish an entire category of bugs forever with this.