Accessibility is much more of a design (to design accessible experiences) and legal challenge (to defend your experiences as compliant in the face of malicious litigation and vague laws) than it is an engineering one. Yes, we can solve the technical problem fairly easy, but that won’t reduce compliance costs very much, if at all.

Why would compliance costs differ greatly once the technical problem of tools that build for accessibility by default are commonplace? Maybe some minimal oversight to verify accessibility, but no much else. Or am I missing something?

You are missing where most of the work/cost is occurring, it isn't in the code, but in the design and verification process.