I'm kind of shocked the physical tooling exists to make a Game Boy cartridge and chips more than anything else.
Though they could have. Limited Run Games does Gameboy cartridges. I'm not sure what kind of tooling they use for it though.
In fact, I'm pretty confident that a competent DIYer could 3D print a cartridge shell, flash a ROM, and etch a PCB in an afternoon or two.
For games more complicated than a simple ROM chip, the Game Boy presents an interesting challenge: the cartridge is both very small and it needs to be able to run off battery power. That excludes using both a high-power-consumption modern SoC or FPGA, or using lots of discrete logic.
But if it'll fit in 32 KB, it's 3D print a case, order a PCB, and program a standard Flash chip.
We've actually come full circle on this one. If you ever disassemble a Nintendo Switch Game Card that has a black area above the metal contacts, it doesn't even have a PCB. It's just a ROM chip, with metal contacts on the bottom shaped differently than your standard BGA pattern, contained in plastic. If it has a green area above the metal contacts, it's a PCB with the chip on the other side.
You can get brand new Atari 2600 cartridges in boxes and everything.
Though, I think the shells are actually scavenged from old games.
Yup, new titles still being made. There's even Halo for the 2600.
Tons of titles currently being manufactured for a lot of vintage consoles/computers: https://atariage.com/store/