But there are a couple of reasons why I hire for for expertise in the ecosystem. Granted I'm in .NET which is a very homogenous ecosystem, so framework/library/tool knowledge bifurcates into people who are experts on every tool/library/framework we use and people who've never used one.
1. For the first 6 months you're paying a sr. engineers salary to get jr. engineer productivity. Given that most devs switch after 2 years this is a huge productivity hit and it's all front loaded.
2. While great devs are great, it's a lot easier to figure out if someone is a framework/language expert than if they are an intrinsically great dev in an interview. i.e. ("Do you know C# well?" is easier to determine than "Are you a good problem solver?")
3. Being an expert on an ecosystem pays dividends even after the first couple of years albeit smaller than the first 2.
4. It's not that much harder to hire someone with expertise in your ecosystem than not.
5. Most devs are searching for jobs in the ecosystems they like working in. But a lot of devs who can't find a job get desperate and start applying to everything, which can bring down average quality of people who don't have expertise.