Agencies do not exist to build great software. I mean sure, it helps if their output is good enough to guarantee more work, but that's not why they exist. Their purpose as a business is to generate billable hours.
Customers do not hire agencies with the expectation they will build great software. They turn to agencies because they don't have the time or skills to do it themselves, and agencies are cheaper than hiring people and they are more dependable than freelancers. By that I don't mean that freelancers are collectively unreliable, but with an agency you have a dozen or more developers and if one falls sick, they can just replace them with another developer. If your product depends on that one freelancer, you are screwed.
So "10x developers" aren't really suited for agencies, where every hour is billable and accounted for, and the output does not have to be great, it just needs to be more or less reliable and more or less on time and under budget (or at least cost overruns are accounted for).
Customers do not hire agencies with the expectation they will build great software. They turn to agencies because they don't have the time or skills to do it themselves, and agencies are cheaper than hiring people and they are more dependable than freelancers. By that I don't mean that freelancers are collectively unreliable, but with an agency you have a dozen or more developers and if one falls sick, they can just replace them with another developer. If your product depends on that one freelancer, you are screwed.
So "10x developers" aren't really suited for agencies, where every hour is billable and accounted for, and the output does not have to be great, it just needs to be more or less reliable and more or less on time and under budget (or at least cost overruns are accounted for).