Probably because once you start adding things up the accessories will be significantly more than the gun.
Take a Sig P226 9mm with a threaded barrel for instance, you might be able to get one for $800 or even $700 but more likely $900.
Now, suppress it.
$200 for the stamp
$500-$600 for the suppressor
$125-$150 for the Nielsen mount
Assuming you paid anywhere near MSRP you just shelled out $800-$1000 for a legal 9mm suppressor.
Put it on a $250 weapon? The stamp alone costs almost as much as the gun. Of course if you had more than one gun to host it on then who can blame you for trying to maximize the use of your toys.
Clickety-Pop for bigger
On threading the P11 barrel. You might can get someone to thread the existing barrel and add an adapter for $100 to $150 depending on who you use and if it's even possible. The KT barrels have a pinch at the end and it's not like we're talking about the thickest tube on the market so I don't know if anyone would be willing to thread and adapt an existing barrel even if it were possible.
But, it's easy enough to find out and it doesn't cost anything for the answer. All you have to do is find someone in your area who threads barrels and let them take a look.