The commentators point out that younger representatives had to be sent in order to be able to travel and examine the land. It was like a congressional delegation sending the appropriate staffers to report back what they found.
Rav Hirsch explains:
These were not the tribal princes, the נשיאי מטות אבותם, who are
called ראשי אלפי ישראל, ראש לבית אבותיו in Ch I,4 &16, the
official heads of a state division, but each one was נשיא בהם, they
were בהם, not the head but amongst them; but in their characters
and abilities they ranged above the masses, were נשיא בהם. Although
they were not the official leaders, they were as it says in the
following verse ראשי בני ישראל, popular heads, men of leading
influence among the people.