Why are most recommended books by computer scientists rather old?

Because most of the essential concepts in computer science were developed by the early 70s. In addition, the concepts were developed in an era of scarce computing resources. Acess to relatively unlimited resources does not foster creativity.

The phrase "Necessity is the mother of invention." comes to mind.

Look up the youtube demo videos (from the 1960s) of:

* Sketchpad

* The Mother of All Demos

* Grail

The old books are still valid because in the words of the old sayings: "Don't re-invent the wheel" and "If it works don't fix it".

If you think those sayings are not correct, merely look at the various Hollywood re-makes of hit movies of the past. Almost invariably, the older versions were much better than today's re-makes.

Knuth is still writing TAoCP, and it is older than any of the books you mention.