I mean you are basically correct that there are people who simply don't/won't vote one way or the other. For context I think it's worth pointing out US participation in the 2024 election in terms of percentages was consistent or better than the participation most countries get in their elections, it's not just a US issue. As for why people don't, I don't really have an answer there. Some people just can't be fucked. Some people's material conditions are bad enough with multiple jobs and responsibilities they don't have the time of energy to devote to it. Some people are totally oblivious that it's happening.

A darkly amusing anecdote is that one of the most searched terms on election day was "is Biden running" or something like that if that gives you any context how disengaged some people are.

That said, there are an increasing number of people in the US who don't vote because no one on the ballot comes even close to representing them or their interests. Not to say that they are both equally bad or equally distant from what they would prefer to vote for, but as Democrats move demonstrably more to the right with each passing election, there has to be a line somewhere at which point they are so far from your interests that they don't get to just count on you to vote blue no matter who anymore. For many people, that line was Democrats enthusiastic support of genocide.

To further the frustrations, it was made abundantly clear through polls and campaign strategists that support for Israel was an unpopular stance among democratic voters, but Kamala gambled on trump being scary enough she could ignore those voters. Worked out real well for her /s