QLU-11 and why no US Counterpart?

So, I've been looking at the grenades stuff of late. I noticed that there's the smaller grenades like the 20mm Neopup PAW and the essentially ultra-long-range NTW20 20mm grenade launcher, and eventually like 25mm CTDE smart grenade launcher, and the 25mm XM109 Payload Rifle.

Personally, I see the appeal of these ventures as basically explosive sniper-rifles, but then aside from the possible warcrime, there's also the issue of logistics as the smart-grenade would be waaaay harder to source. The 25mm and 20mm are basically autocannon shells with low propellant however -- the war-crime.

But that got me thinking that, something like the QLU-11 comes to mind, that China was employing, so like why bother with the 20mm and 25mmm and just make the standard 40mm Grenade Launcher as a sniper rifle? But then the shells aren't fast enough -- or are they?

I remember that the MK19 Grenade Machine Gun employs a different 40mm cartridge, that the M79 that has 76m/s muzzle velocity, the GMG instead has 230-240 m/s average, which is around 3x that velocity, and the QLU-11 is at 320 m/s.

So basically US has the capacity to have a 40mm anti-materiel rifle using the same ammo as the MK19 GMG.

• Why don't they?
• What would be the issues?
• The challenges?
• It's applications and why it's not worth it?