Mostly history, but also, depending on what you do, you may need space for bearings/bushings.
Once upon a time, weightlifting was new and the quality of steel you could get wasn't that great, especially when you needed hundreds of pounds of it. The Victorian weightlifters didn't buy those cartoon, fixed weight, globe-end bars because they hated the idea of being able to adjust weight, they did it because if they didn't, then things would break.
Eventually junk level steel progressed to the point where you could make plates and sleeves that would hold up, so long as the interface between them was around 2" in diameter. But up through the 1950s the bar in the barbell would occasionally break. The solution was to wait until good enough steel became cheap enough to make the bar (and only the bar) out of that. Eleiko was the first to do this in the late 50s, and those bars were expensive. Despite most of the steel being cheaper stuff (also, tooling for better steel costs more, takes more time, skill,...).
These days good steel is cheap and common enough that for puny humans that only lift up to around 1000 lbs, the "standard" model would be fine for many movements (ok, 28 mm vs 1 inch is actually kind of important for some human beings. But not many.). But at this point we have something like 100 years of standardization on the 2" model. It's the one that's defined. It's the one that's made by quality manufacturers,... Companies could make a 28mm bar and plates that were well made, calibrated, etc, but they're not going to, because the "standard" bars and plates have a reputation for being low quality, breaking, etc. And at this point, the reputation is self fulfilling.
Note that I said "for many movements". If you're doing powerlifting, you don't care about how well the sleeves spin on the bar. But if you're doing olympic lifts, you care. So you need bearings or bushings between the bar and the sleeve. These don't have to make the sleeve 2", but that's what we're used to, so why not?