Comment by esics6A

Because you're looking for a very specific language or tool experience and not capabilities. This is a common mistake. Many developers might have loads of experience in the problem domain your company needs but not a specific language or framework. Like you do integrations for banking clients. You look for a Java developer and find someone who knows the framework you're using but they've only ever developed APIs for mobile apps. Instead look for the person who might have done some Java a lot of Cobol and has worked with mainframes and banking systems. That person might have even been a sysadmin and learned coding but they'll hit the ground running.