The problem with sandboxing is that you're limited to the APIs that your vendor provides, which stifles creativity. For a vendor to open those APIs up, they have to realize that there's somebody out there wishing to use these APIs. If all platforms were properly sandboxed from the start, a lot of software we now know and love would never have existed, it couldn't be created because of the missing APIs, but the APIs wouldn't be added because the vendor wouldn't know that some software needs them.

Screen readers are a good example, proper accessibility APIs came later, as a response to screen readers' needs. The first screen readers used various tricks for injecting code into other processes and emulating GPU drivers to intercept GDI calls.