Creating a minimalist server is an interesting excercise, everyone should do it to see how it's done imo. BUT it's the stuff like parsing http headers, query parameters, and request bodies (formencoded, chunked files?) that make a solution like Jetty worth while. Again, writing an http parser is a valuable learning experience, but your boss really doesn't want you spending time on it, OR paying to support it going forward.
As an aside, it used to be a thing in the late 90's early 2000's for every application out there to have it's own "XYZ Transport Protocol" ie XYZTP, and it really, really sucked.
That's really the benefit. I have been using jetty for years. I trust them a lot more than I trust myself to create a web-server. If I were to write everything from scratch, I would never have finished the project.
About a decade ago I worked on the query processor component for the Yahoo Vespa platform and I spent a month writing a web server to squeeze as much performance out of the JVM as possible. And indeed it was quite a bit faster than Jetty. Speed being a bit significant when you have a stringent latency budget.
However, I ended up deciding it wasn't really worth it. The extra millisecond or so consumed by Jetty wasn't worth the cost of having to maintain a relatively complete implementation of HTTP. So I took out my implementation and put Jetty back in.