Why do legal documents and government notices use legalese instead of wording people can actually understand?

Because regular language isn't as descriptive as it should be to make an agreement with. Agreements typically define things as either A or B, with no empty space in between to meddle over. Think of it as an agreement with Mom and Dad:

When you finish your dinner you will get to play with the Lego or the other toys.

Now, "finish" your "dinner"? Does that mean I have to stop eating, that I have to eat all vegetables or that the plate must be clean? Do I have to lick up the sauce to count as "finished"? If I finish it by feeding it all to Mom, does that count? And then when I do get to play, do I get to play with the Lego and the other toys, or do I have to choose between the Lego on the one side and the other toys on the other? So we go another draft:

When you finish your dinner, meaning that at least 90% of the vegetables by weight and at least 50% of the potatoes and meat are consumed by you, you may play with any or all of your toys at your discretion.

That's a lot closer to the intended meaning but it's a lot harder to read too. Lawyers typically try to close all visible loopholes.

Then there's another problem; your sister gets a slightly different agreement as she was "grandfathered" into the agreement. Your sister wouldn't accept any new agreement and would insist on what she got last time instead, but for you your parents want to tighten the rules a bit more. Putting this in one agreement is a very hard thing for people to do without tightening the rules for your sister or giving you a chance of leeway where you shouldn't get it.

And then there's the consequence if they get it wrong. Somebody could get around paying taxes (saving millions), sell their house and not pay any sales tax on it, pay really little on road maintenance or drive a car that's legally owned by Venezuela so they only pay $0.02 for gas. All of those mean that there's less tax money and those imply that less environment maintenance / war protection / ... will happen. If you ever wonder why schools can look bad or roads aren't maintained? There's just not enough tax money coming in to do all of them...