Depends how many they need and how much control. Do they want to be a server company or an adapting-servers-to-run-your-CI/CD company or both? You can extract value from both parts of the equation, but theoretical economics tells us you can get the most value for the least effort by doing more of what you're best at and paying someone else to do what they're best at, rather than doing everything mediocrely yourself.
Sometimes that other company isn't actually very good and you can increase value by insourcing their part of your operation. But you can't assume that is always the case. It wouldn't have solved this particular problem - I think we can safely guess that your chance of getting a batch of faulty motherboards is at least as high as Hetzner's chance.
Building and owning an institution that finances, racks, services, networks, and disposes of servers, both takes time and increases the commitment level. Hetzner is month to month, with a fixed overhead for fresh leasing of servers: the set-up fee.
This is a lot to administer when also building a software institution, and a business. It was not certain at the outset, for example, that the GitHub Actions Runner product would be as popular as it became. In its earliest form, it was partially an engineering test for our virtual machines, and we went around asking friendly contacts that we knew would report abnormalities to use it. There's another universe where it only went as far as an engineering test, and our utilization and revenue pattern (that is, utility to other people) is different.
Sometimes that other company isn't actually very good and you can increase value by insourcing their part of your operation. But you can't assume that is always the case. It wouldn't have solved this particular problem - I think we can safely guess that your chance of getting a batch of faulty motherboards is at least as high as Hetzner's chance.