I think I know the answer to this. It's the same as the answer to the following questions:
"Why does my bike shop always add air to my tires so they are like rocks? Without even telling me?"
"Why does my ski shop always try to detune my tips and tails?"
"Why does the waiter always put the white wine in ice water, making it so cold I can't taste it?"
Because businesses go out of their way to avoid doing things that make the average customer complain loudest and most often.
"You worked on my bike and I got a flat halfway through my first ride!"
"You worked on my skis, and I caught an edge and separated my shoulder on my first run!"
"This restaurant is so clueless they don't know that white wine is supposed to be served cold!"
In the case of spacers ... A big lug of a user goes on a test ride and tries as hard as he can to bottom out the suspension. If he succeeds, he doesn't buy the bike. If it bounces back from anything, like a beach ball, it's awesome.
+++++++++++++++++++++++++++
On a less rant-y note, there is, of course a place for spacers. Obviously if you're a big person you might need a more progressive action to keep from using up all the travel all the time. Similarly, if your local riding is, like, 15 minutes of smooth going followed by two three-foot drops, you're going to want your suspension to behave one way. If, on the other hand, you're like me and your local riding is a non-stop latticework of two-inch-high wet roots, you're going to want your suspension to behave in a different way.