Hiring, like many things - including engineering - is about tradeoffs.
Is someone more knowledgeable "better"? Sure. But sometimes it's about getting a specific thing done, and if you're hiring someone to improve a web thing that uses React, maybe you don't need someone who understands compilers and the kernel and how the underlying hardware works and all that. Maybe you can spend a bit less and get someone who will do a perfectly adequate job.
Is someone more knowledgeable "better"? Sure. But sometimes it's about getting a specific thing done, and if you're hiring someone to improve a web thing that uses React, maybe you don't need someone who understands compilers and the kernel and how the underlying hardware works and all that. Maybe you can spend a bit less and get someone who will do a perfectly adequate job.