I realize that there’s no easy answer to this question, and possibly no answer at all. So how about a similar question: What prevents the charge of, say, an electron from spreading out? How is it confined to one point?
For most things in nature there seems to exist a “gradient effect” where things go from high concentration to low concentration. But for fundamental particles (and their properties) this seems not to be the case, ie. although there is a gradient between “particle” and “non-particle”, the particle does not expand out or flow along the gradient. It is somehow constrained.
And, as always, if this question is just a reflection of my complete lack of understanding of such things, and is fundamentally naive and/or misguided, just say so!
Ok, since no one has answered this yet, I’ll give my rookie, ninth grade interpretation.
First of all you can’t really relate macro experiences like a gradient effect to the world of particle physics because the two worlds have completely different laws. It’s pointless to try to compare them on any level, generally. The whole point about being a fundamental particle is that you don’t spread out and can’t be broken into smaller pieces. note: (My namesake is credited with making this notion popular)
Now, depending on where you’re going with fundamental particles, they are apparently held together with things called gluons which carry the strong interaction force. Each fundamental particle has a certain, inherent charge associated with it. Well, some have zero charge, but that’s besides the point. The point is, they have a discrete charge. This is supported by the Law of Conservation of Energy and the Law of Conservation of Electric charge. This is where I get hazy, but it has something to do with not being able to decay, or lose any energy or electric charge because losing one wouldn’t allow the other. I’ll try to go find something on this, as it’s been years since I was well read on the subject.
A fundamental particle holds together just because there isn’t anything smaller for them to spread out into. An electron, for instance, can’t spread out into a bunch of little charged pieces, because there are no charged pieces smaller than an electron. If you’re asking why there aren’t any charged particles smaller than an electron, then the best answer is just that there has to be something at the bottom level of structure. It really becomes more of a philosophical question.
Particles are held together by the exchange of other particles. Electrons are bound to atomic nucleii by exchange of photons. Quarks are bound together (in protons and neutrons, e.g.) by exchange of gluons.
As far as anybody knows, electrons cannot be divided - they are truly a-tomos, undividable. Why? No one knows. Philosophically, it seems that the “bottom level of structure” could be a continuum, rather than lumpy. But it isn’t. The Universe prefers chunky style, we just have to deal.
