They could make an SSO but who would use it? Proton users make up such a small portion of the web that it wouldn't be worth the effort and they might as well include 8541705317 other accounts at that point, as far as they're concerned.
They use SSO because almost everyone already has 1 or more accounts with Google, Amazon, Microsoft, etc.
@helenslunch Nobody uses a good privacy/security focused IdP because one doesn’t exist for consumer / smb besides a small handful (many which you noted).
No, no one uses them for the same reason they don't use any other privacy/security-focused product: No demand. There's a small niche of consumers who do care and use products like Proton but it's an absolutely miniscule fraction of the populace who simply don't care, or don't care enough to actually do anything about it.