The high-level cause is that the system is set up in a way that macro-characteristics of each letter are more important than the appearance of a given letter at a given screen resolution. The idea is that a given section of text should have the same length (for a given scale) regardless of the pixel density. I believe TrueType fonts are this way.
The opposite of this is the Apple “screen font” system (predating TrueType), where the characters for a font are designed by pixel maps, with different pixel maps for different font sizes. Of course, a screen font generally doesn’t have a pixel map for every possible size, and when you use other sizes, it has to interpolate, and this interpolation is considerably worse than the aliasing that TrueType fonts get. (Note: I don’t know the internals of TT fonts, and I wouldn’t be surprised if it allows different types of specifications. But my understanding is, it has a primitive for a section of a curve – sort of like a French curve only with thickness too – and you build letters with combinations of these parameterized curves.)
The benefit of this system is generally better-looking fonts for a wider variety of font sizes, including non-integral sizes, and the property that whenever you scale the font, it remains true to scale, but with the aliasing artifacts (which are worse at some scales than others for reasons I don’t know but I suspect it has something to do with “screen hints” being absent for some sizes. I never found out much detail about screen hints. I abandoned making or fiddling with fonts after taking a stab at TT, which requires a lot more dedication, skill, talent, and time than designing simple screen fonts in a few sizes.)
My background with fonts is that back in the 70’s I made one for a PDP8 that used a storage oscilloscope as a screen, and later in the 80’s I made very small but highly legible screen fonts for my personal use on my Mac Plus, fonts I continued to use into the 90’s. So, I’m just a font dilettante. But I did bump up against issues like TT fonts and kerning and all sorts of other wizardry. They’re surprisingly sophisticated! The great Douglas Hofstadter did a lot of the early groundwork for the art and science of computer typesetting, of which fonts is a subset.
I shudder to think of how hard I’d have to squint to read my 6- and 8-point fonts with today’s pixel density (not to mention my well-over-40 eyesight). I can say with great confidence that they were far more legible at those sizes than any fonts I’ve seen since. No doubt they’d make a serious font designer chuckle, though.