Professionals have large networks of neurons. They are sturdy and efficient from repeated use. Memory palaces help to start the construction of these large networks of neurons. Afterwards, as another commenter noted, the knowledge is deeply processed. Mnemonics are replaced by networks of meaning. It is no longer “This algorithm rhymes with tomato”, but “This algorithm is faster if the data is stored in faster hardware, but our equipment is old so we better use this other algorithm for now”.

Broadly, the progression of learning is: superficial learning, deep learning, and transfer. Check out Visible Learning: The Sequel by John Hattie for more on this.

Edit: To directly answer your question, experts have so many sturdy neural hooks on which to hang new knowledge that mnemonics become less and less necessary. Mnemonics may be particularly helpful when first learning something challenging, but are less necessary as people learn.

You could also check out a paradox called the expert paradox. We used to think memory is boxes that get filled. This idea was directly challenged by Craik and Lockhart’s Levels of Processing. Levels of processing supports the idea that “the more you know, the faster you learn”. Note that this is domain-specific. In other words, an expert in dog training won’t learn quantum mechanics faster than anyone else.

At least to me the use of memory palaces actually precludes the use of the stored information at the conceptual level.

For me the usefulness of memorized information is the concepts it links to elsewhere in memory. Memory palaces replace these conceptual links with the "physical" link to the location in the memory palace reducing the information's actual usefulness. So what is produced in my mind is a linear list I can recite, but those words have no meaning conceptually.

For me, memory palaces work great at a party trick, but information is useful to me in conceptual form with its many links to other existing concepts I have, not as a rote list.

I've read a comment by someone who put a staircase in their memory palace with a step for each entry on the periodic table of elements. Anecdotal, I know.