Resolution, generally.

A laser printer operates by using UV light to make fine pigment powder stick to a drum by static electricity. True to it's name, it used to be done via a laser that scanned the drum by reflecting off a rotating mirror - but nowadays it's just as often a line of tiny UV LEDs. The pigment is than baked onto the paper by a small electric oven.

The pulses of the laser and the pitch of those LEDs is generally way finer than what your run of the mill 3D printer is able to achieve reliably. And definitely finer than any nozzle you could put onto a 3D printer.

Theoretically you could DIY the spinning mirror approach, but it would be difficult to source the optical parts, and calibrating it would be a gigantic pain in the ass. Not to mention that it would likely be significantly more expensive than an off-the-shelf laser printer.
Also, guess what happens if you don't have toner cartridge and print drum as one sealed unit. The printing medium is so fine it gets everywhere, ask anyone who ever tried reloading one of those cartridges.

Square Singer explained the difference with InkJet above.

Modern paper printers are deceptively advanced machines. They'd be pretty impressive if not for the greed of the manufacturers. High-precision parts made just right so that you could print out whatever annoying document your employer wants you to actually sign and bring in physically.

A 3D printer is comparatively slow and generally prints in one colour. As I said, you can make a plotter easily by swapping out the print head for a pen, but then you have a single-colour printer that's significantly slower than modern laser printers, that can be upgraded to have multiple colours with a toolchanger but won't produce anything near the resolution of an inkjet (or even a laser printer, tbh).

For reference, this is how a plotter at work looks like. Similar to bed slingers, ain't it.

I feel like theoretically it maybe could be possible to turn an SLA printer into a paper printer, with resin solidifying on a page? But then how would you keep the rest of the page from being smudged?