The same thing happens with rock bands. Sometimes they tried to put together bands called "supergroups" with star musicians from other highly-successful bands, and the results were usually underwhelming. It's probably because many times, one of the "underperforming" musicians does various other important tasks that don't involve technical ability on their instrument. One example here is Nikki Sixx from Motley Crue: he's a competent bassist for that type of music, but definitely no virtuoso, but his big contribution to his band was in songwriting, not bass playing, and his songwriting is arguably one of the big things that made the band so popular and successful in its time.
Also, sometimes certain people just work really well together, and one of them may not seem all that great on their own.
Also, sometimes certain people just work really well together, and one of them may not seem all that great on their own.