This is just my opinion and guess, but I have noticed a lot of jobs for frontend development require knowledge of WordPress; a lot of businesses (not major ones necessarily but even some of them use Wordpress) use WordPress so it probably helps to know some basics of it (I've thought the same as you but yeah, like I said, I have seen quite a few jobs ask for knowledge/experience in WordPress so I can see why it's good to know some of it).
Why WordPress why not Headless CMS?
This is just my opinion and guess, but I have noticed a lot of jobs for frontend development require knowledge of WordPress; a lot of businesses (not major ones necessarily but even some of them use Wordpress) use WordPress so it probably helps to know some basics of it (I've thought the same as you but yeah, like I said, I have seen quite a few jobs ask for knowledge/experience in WordPress so I can see why it's good to know some of it).
Hope that helps some, good luck!!
Thanks for your response, good luck!