If you're talking about ground lines, the poles that are right by the base of the jump, then trainers may not think it's necessary to add those in when quickly setting jumps in the warm up, although groundlines do help horses (and riders) see the distance a little bit easier. The trainers may have a specific purpose in building an airy jump without a groundlines, however, such as trying to force a rub to make the horse more careful before heading into the jumper ring.
I've never seen a groundline set as far away as the jump is high.
Almost all groundlines will be at least 1'6" away from being directly under the top rail, though.
If you mean a ground pole set any certain distance from the jump, then these are being used for any number of specific purposes, so other trainers may not need to address those things when prepping their horses/students for the ring, and that why they're not using them. You will never see those in the show ring.
At rated shows, hunter classes will have groundlines (as well as boxes, tons of fill, etc) at all of the jumps (except in some derbies and higher level handy classes, where you might have one true vertical with no groundline) but jumper classes will almost never build jumps with groundlines, because the jumpers are asking different questions of the horse and rider than the hunters are. At lower level and unrated shows I think you will sometimes see hunter classes without groundlines at the jumps, but that is more because of a lack of resources/knowledge in course design at those shows.