Why does the Excelsior class appear everywhere?

Why do Excelsior-class ships seem to turn up everywhere?

Because Starfleet appears to have kept building a successful, adaptable design in large numbers, much like the long-running Arleigh Burke program.

The discreet Galaxy program contrasts with the seemingly continuous construction of Excelsiors. Their moderate size and repeatable design make them analogous to Arleigh Burke-class destroyers or the Royal Navy's far-flung cruiser squadrons. The result is a class numerous enough to appear wherever a regional force is needed, even when senior command has lost track of every deployment.