Apathy, or they find more value in spending their attention and time doing something else, a feeling like their vote won't change a thing, or straight up laziness.

I don't actually think it's important why someone doesn't vote, so long as their right to vote isn't impeded. I have a theory that society, like many other systems, tends to take on more stable forms over time. Part of the reason democracy has been so stable is that it allows people, who would otherwise be unhappy and want to change things, a chance to have a say. Non-voters are already a "stable" part of society, since they apparently don't have strong enough feelings to go vote for something different. I don't see a problem with that.