If you have independent votes on everything then you run the risk that there are a bunch of mutually-incompatible things that all get majority approval.
(Would you rather higher taxes or lower? Lower, of course. Higher state pensions or lower? Higher, of course. Stronger or weaker military? Stronger, of course. Better or worse infrastructure? Better, of course. More teachers or fewer? More, of course. More national debt or less? Less, of course. Etc.)
This doesn't require any individual person to be irrational or forgetful or anything, although in fact people frequently are.
Also, whoever selects just which things get voted on has a great deal of power, more than most elected representatives have. If those people are elected then you've effectively got a representative democracy after all; if not, then arguably you've effectively not got a democracy at all.
Representative government as such doesn't solve this problem, but in practice it means that a candidate or party proposes a whole basket of policies to get judged collectively, and between when they get into power and when the electorate decides whether they did a good enough job to elect them again there's enough time for a wide variety of those different interacting things all to have happened and either worked well or not.
I don't want to claim that this works particularly well. But it feels to me like any sort of direct democracy would likely work much worse.
(Maybe there's scope for a hybrid system: elections every few years for representatives who are then obliged to put various classes of major decision to a national vote.)