Durability and precision is an issue at the size you need.
Can you make one, yes, will it break as soon as you put it on the stem, quite possibly. Keycaps are injection molded at high pressure, creating a very dense plastic, 3d printers lay the plastic layer upon layer, not only is it not dense, but there are micro fractures between the layers. Resin is better in this aspect to a degree, however the resin has less strength than the ABS and PLA typical printers use. You could use casting resin and some molds and get much stronger parts than 3d printed parts.
If you really want 3d printed caps, your best bet is to have them done in metal by a company like Shapeways, since, even less dense metal is stronger than layered plastic. Keep in mind, one teeny mistake, forgot something, or just figuring out tolerances (which are very important here!), all cost money. You may be thinking you can afford one, but it may take 5 or more tries before you get it all figured out. What you could do is have them do one in plastic(about $4 ea), get it worked and then switch to metal ($20-$30 per cap), however, the tolerances change from metal to plastic and in 3d printing it can cause some very strange effects. Worse, with the type of machine Shapeways uses, you may not be able to account for it, because of the style printer, the direction of warp or distortion may be different each time you order.
Just because you CAN 3d print something, doesn't mean you should.
So why and how do people do it? Because they can. It's cheap fun, and a challenge, things like this help you understand your printer and helps with other projects, think of them more as a constructive/engineering exercise than making practical key caps.