At a busy airport and/or with a control tower the requested pattern can often take you too far way from the runway to glide in without power. But there is no reason to descend early. Doing so annoys the neighbors and leaves you vulnerable to wind shear and a mechanical or fuel burp. Better to stay high.

Those of us who fly airplanes that glide like manhole covers know that the intent of practicing power off approaches at an airport is to keep fresh our knowledge of glide performance. Engines are reliable so it is easy to let that knowledge get stale. Repetition keeps that knowledge in a place in our brain where it is right there when needed. Kind of like why the every 6 month's sim rides that Part 121 crew have to do seems to be a series of emergencies rather than just easy hops where normal procedures are demonstrated.

"Why do we pretend the traffic pattern is the only place..."

There are some places where you can practice your power-off approaches to farmer's fields or dirt roads. Where I live in the woods, I don't think that's practical. The local flight schools have their students aim at the private strips, like the one I live on. But that's as good as it gets. If they really do lose their engine they will be demonstrating their tree-top landing skills. Of course, on their long cross-country flight they will see geography that is friendlier to sick airplanes.

Best of luck,

Wes