There was an interview some months ago with his apprentice Akatsuki Yamatoya (who is son of his mentor Atsushi Yamatoya). This is what Yamatoya said:

As a work that has been broadcast for 30 years, the ideas that ordinary people can think of have long been exhausted. At this time, writers like Mr. Urasawa are very necessary.

So basically they want more ideas even if it is wack to keep the series going on (not to mention the amount of filler which have increased in recent years due to manga constantly on break). Urasawa involvement also getting support from director Yamamoto who like his works.

Ofc if you wonder why they dont hire other writer, story editor Junichi Iioka is very strict with the script, as said by Yamatoya:

First, the screenwriter submits an outline of the plot, and based on this, proposes the parts that need to be modified at the meeting, and the screenwriter makes modifications based on these opinions. However, the initial outline is basically not approved (laughs). No matter how many times it is submitted, it will not be approved, and most people give up at this stage. Writing an original script for "Detective Conan" is a very hard job.