And why neck down a 40 and then try to get it to expand back up to 40?
How silly is that?
Well, for those of us who have carried for a living, the necked-down cartridge makes a ton of sense. It is a far more reliable feeder in a semi handgun than any straight-walled cartridge.
And your 9mm bullet makers are constantly trying to do the same thing - get their pills to expand to 40 or 45 dimensions, agreed?
So the better questions are:
Why would you consider the 9mm the better performer when, in fact ...
1 - SIG 357 and 9 mm are both .355
2 - SIG 357 is loaded hotter and still remains within acceptable SAMMI standards.
3 - SIG 357 is a far more reliable feeder and therefore far less prone to stove-piping.
4 - SIG 357 is a far more accurate, flatter shooting tack driving load.
5 - SIG 357 is a far better penetrator, especially against glass and heavy clothing. It's just a better overall, all-purpose pill for the serious shooter who carries for a living.
... and there is no denying any of the above. There are multiple sources available that support my contention.
The two advantages 9mm holds over SIG 357 are higher capacity mags and less recoil. And I typed all of the above as a true advocate of 9mm (which I carried for many years), especially with modern pills. But still, 9mm does not compare favorably to SIG 357.