Because it's explicitly against the rules of the site! See #4 in the help section "Asking"[0]

[0] https://stackoverflow.com/help/on-topic

The code is ideally a "this is the code that fixes the problem" and its done. Once there's an answer, you don't often get a dozen more answers with slightly different code.

Library recommendations however - everyone has their favorite. Someone recommends Spring. Someone else recommends Play. Someone else recommends Stripes (and then has a dozen comments explaining the different between Stripes and Stripe). Someone else has Struts (and then gets a dozen comments about the version they're recommending being out of date and lead to the equifax breech). JSF, Jspx, JHipster, Grails, GWT, ... it goes on and on.

And now that there are a score of answers, some new user sees the post, doesn't read everything and recommends a new version of Spring. Or points to a different site for Struts.

The curation of such a question takes far more work than the question is worth. Look at how much effort the C++ community puts into maintaining the C++ book list. Spending that much effort on each recommendation question is far too much moderation time across the community.

On the other hand... go to a site that is designed with exactly that goal in mind. https://www.slant.co/topics/40/~best-java-web-frameworks does it much better than Stack Overflow can.

But there are times where I want to see everyone vouch for their frameworks in that exact manor in the stack overflow format by the stack overflow crowd.

That rule is in place because the stack overflow crowd doesn’t want to debate it and keeping it useful for anyone else is too much work.

The sibling site to ask for software recommendations is https://softwarerecs.stackexchange.com