I can tell you why: reliability. Amazing handguns and wouldn't I love an HK416 for my collection.
I've well over 5,000 rounds through my HnK USP9 and it hasn't jammed once. Still accurate as all get out too. It's my absolute favorite weapon.
My HK45 just had its first 1,000 rounds put through it without a hitch. Same for my VP9 Tactical, VP40, P30SK, HK45 Tactical, HK45 Compact and my Mark23.
Not a single glitch in any of them. Not one. I also inspect my ammo which no doubt makes a difference too.
I’ve had similar results with my SIGS. I have a P220 in .38 Super that is the only gun in that caliber I’ve owned that has never malfunctioned and eats all ammo.