Most of that comes from serious riders, who spend serious money to get a seriously good, long-term ride.

Now, a longer stem is generally just a band-aid fix for a poor fit; it CAN BE part of the fit process, but your bike has to be close to right to start with. More than 20mm of stem change, and you're on the wrong size bike.

Now, a casual rider can make use of a stem change to salvage an otherwise acceptable bike; since the casual rider doesn't go out and do 50-milers, club rides, centuries, or charity events (usually), a well-fit bike is less of an issue. If you like your bike aside from the stem issue, you likely fall into this group.

It's an inexpensive way to try; you can always keep the extra stem(s) if you have to buckle down and buy new. (I have about 4-5 extra stems laying around, just in case a need arises; that, and they're sweet pieces of alloy!)