We would get contacted by the client to resolve an issue on our end, and after some debugging we would consistently discover it was broken because of the other company. They couldn't produce code that worked.

This could have been a management problem. For example maybe they simply did not care enough about the project to allocate the necessary dev resources. But it also could have been resolved on the dev side if they did a better job of testing their code that they claimed was complete.

Edit: I guess the most direct answer would be, since it was a problem that I could have resolved with tech, I didn't see a reason why they couldn't resolve it with tech.