Because those experts haven't been able to craft unambiguous laws that aren't up for interpretation. There's nothing stopping congress from deferring to those experts still.

If Congress lacks expertise on a particular topic then they can front load the process and seek expert input, then write that into the legislation. There's no need to delegate that to the Executive branch for interpretation after the law has been passed. This might slow down the pace of legislation, which would be fine.

The current pace of legislation is glacier slow

Which is fine. Legislation shouldn't be rushed, and the default should be to do nothing.