Ask anyone who has tried to run banking apps on GrapheneOS how that works out in practice.
GrapheneOS supports attestation. GrapheneOS even provides the sort of security guarantees that would make risk management types at banks happy, but it isn't popular enough for them to be motivated to support it as an attestation target.
Now imagine it was practical for websites to require attestation from browsers. How likely do you think it that all the major services would accept anything other than Chrome, Safari, and Edge?
GrapheneOS supports attestation. GrapheneOS even provides the sort of security guarantees that would make risk management types at banks happy, but it isn't popular enough for them to be motivated to support it as an attestation target.
Now imagine it was practical for websites to require attestation from browsers. How likely do you think it that all the major services would accept anything other than Chrome, Safari, and Edge?