Even if most browsers don't check, it takes just one single person to expose the whole operation. This provides a strong incentive to use compromised cert vendors only for the most important operations, instead of indiscriminately against everyone: each use of a counterfeit certificate risks "burning" the whole cert vendor.

And as you say, browsers today might not check these things (and checking CAA doesn't make much sense for a browser, it's intended to be checked by certificate issuers). But future browsers, browsers customized with some third-party extensions, or non-browsers might check.