When I see someone say something like this, here are my thoughts on the matter:
1) You've been really damn lucky. You've worked with horses who happen to fit reasonably well into the saddles you've made--in some cases, not unreasonable.
2) You haven't tested enough.
Or,
3) You don't really know how to check for soreness in a horse.
I have seen horses hurt really, really badly by poorly fitting saddles, both Western and English. The one hurt the worst was one who *screamed* at me when I touched his back; he'd been ridden for pleasure in a Western saddle only.
I've seen some really nice saddles with lovely trees which just did NOT fit on the horses who were supposed to be wearing them. I'm looking at horses ridden for pleasure or performance, often in fields some ways from where their breeding lies (ie, the downhill-built Paint that they're trying to train in eventing, or the Standardbred that they're riding on the trail, and so on). People love their horses and just want to ride them and enjoy them, and in the main they just don't have a CLUE what the inside of their saddle looks like. They don't know how to check for fit. Getting something approximately right improves these horses' lives a hundredfold.
I've also seen dramatic, immediate differences when we put something that does fit onto a horse. That's not something that could be faked with a con game. Horses who "take 20 or 30 minutes to work into the bridle" and are stiff until then, suddenly are soft and responsive as soon as you mount up. Horses who wouldn't stay in the trot suddenly trot with ease. Horses who wouldn't stride out or have a choppy stride suddenly have a long beautiful stride. This is easing pain in the horse, not some kind of mind game that I play on the riders. Sometimes a small change makes all the difference; sometimes the saddle needs to be changed completely.
It makes me sick to see a horse who's been (or is being) ridden in a badly fitting saddle. It's worse when someone who *makes* saddles doesn't seem to understand why saddles need to fit.