Why does uranium not yield energy if fused?

Answer 1

Because it's impossible. 2 Uranium nuclei forced together would form a highly unstable state that wouldn't stay together for any time at all.

Answer 2

Fusing light elements and fissioning heavy elements are exothermic. They give off energy.
The mid-point is iron. Fissioning or fusing iron takes energy.
It is thought that all the heavier elements were formed in super-novae from lighter elements.