What's different? Basically the whole thing.

A 3D printer (talking here about FDM because SLA really shares nothing at all with a 2D printer) is basically a tiny hot glue gun being moved on three axies by stepper motors. Of course, the temperature and extrusion controls are much more accurate than a hot glue gun, but that's the basic principle. You got a single "printing point" that gets moved around and it only extrudes filament from that single point.

An inkjet printer has one stepper motor that moves the paper and another that moves the print head from left to right. So there too are axies moved on stepper motors. A very simple trait also shared by e.g. CD and disk drives, slot machines, camera lenses and many other things. All these things are as close to a 2D printer as a 3D printer.

The real magic of an inkjet printer is the print head. A print head doesn't have a single nozzle but an array of many nozzles. This way, a printer cannot only print one dot at a time, but instead a few lines at a time. These nozzles are much tinier that the nozzles on a 3D printer, and they also are much more complicated to operate.

A 3D printer just uses a stepper motor to push filament into the printhead, where it melts and is then pushed out of a hole.

On an inkjet printer, you need to either rapidly boil the ink, so that a single vapor bubble appears that pushes just a tiny drop of ink on the paper, or you have a tiny piezoelectric transducer that creats a vibration that then pushes out ink.

This is orders of magnitude more difficult than a 3D printer, and much tinier. You won't be DIYing a working 2D printer from scratch, while that isn't all that hard for a 3D printer. With access to a decent toolshop, you can make all relevant parts of a 3D printer. The same is not true for 2D printers.

To rephrase your question: Why is it that so many people build DIY desktop PCs, but nobody is making a DIY flagship smartphone? What's the difference?

Basically everything.