There are a few layers to it.

On the surface, the excuse they give is that printed guns don't have serial numbers and thus can't be tracked. They call them "ghost guns" to instill an element of horror in the messaging.

If you dig a bit deeper, you'll see that it's the same old tactic of manufacturing outrage and spreading fear in order to manipulate the masses. Even when people supposedly see through it, on the collective level it still works. So it seems to be an effective tool, which encourages them to keep using it.

On a deeper level than that, it's a trojan horse. Just one more way to restrict consumers rather than holding corporations accountable. One more way to maintain their hold on people's wallets. 3D printers give people too much autonomy, enabling them to be self-reliant. The people benefiting from the current system can't have that, they won't tolerate it. "No, you'll spend $50 on that action figure, and if you need ten of them you can either cough up the dough or go to hell." Basically.

It's not about firearms.

It's about controlling what you can 3D print.

When your 3D printer has to connect to a third party service to check if it's allowed to print what you just sent it. That's a clear vector for companies to enforce IPs.

Printing a replacement part for your appliance? Sorry, they're blocked.

Printing parts to repair part of your vehicle or snap something back on? Sorry, that's banned.

Printing something that resembles the intellectual property of any other company? Sorry, that's banned.

Can't have you cutting into the profits of corporations by self-servicing and self-repairing.

Also a mass surveillance device to produce surveillance of what people are 3D printing and report it to a central authority.

Because it makes for a good distraction from actual problems that they don't care to solve because those problems would require them to heavily tax millionaires and billionaires.

See Luigi.

It's not the gun, it's gun makers worried about DIY gunsmithing. The politicians are just the bitches.

Also not from America, but I assume this is just the same sort of thinking as trying to ban trench coats after the Columbine shooting, IE: they have to look like they're doing something, but they can't do the actual thing that would solve the problem because there's too much money involved and they're greedy cowards.

Also it's weird that Teen Vogue was the best source that came up when I searched for that lol.

Because it makes firearms available to people without having to jump through hoops the government can track, but they can make a machine that makes flexi-dragons into a boogyman, so they throw a "protect the children" in the bill and it automatically passes.

Because it doesn't make money for Big War

Just like all the "for the kids" bullshit excuses to enact draconian population monitoring, this system will make it harder for small manufacturing shops to compete with the mega corps.

Because money. Firearms are everywhere in the US because of gun lobbyists. If citizens print their own guns then money is lost.

Because our government is run by old dumbasses.

Because between them, the legislators don't have two brain-cells to rub together and figure out why this is an un-enforceable bunch of bullshit.