While I too wish it was able to be disabled easily, I'm surprised the parent poster is getting down voted. Having worked for hardware companies, and chip companies in particular, I can very easily see the benign answer being that this isn't even on their radar as something to do. That they are providing the ME features because Sales Guys say that some big enterprise customers want them, and once it was designed in, there wasn't a convincing proposal to make it an optional / opt-in feature, because there simply isn't someone on the engineering team who is voicing the concerns of the "home PC user who cares about this-kind-of-thing."
And even if there is, the "non-evil" reason for keeping this as is might be that "enterprise customer doesn't want something that is easily disabled, and we can't justify cost of a separate design / fab run for a small minority of customers", vs. "Haha, now we have a backdoor into EVERYTHING!"
(Hell, even the proposals about disable via hardware switch, or having the ME on a separate component probably boil down to money. To an outsider, hardware companies are funny beasts in that they really start caring about things like counting pennies and minimizing BOM cost as much as possible. I've been in worried meetings where we were literally counting and debating pennies, and wondering how I got there, but it makes sense at a company level since those pennies add up when you are building thousands or millions of something! This may have boiled down to "motherboard manufacturer doesn't want to have to put another mechanical part on the board, and they are looking to buy a shitload of our parts, let's keep them happy")
Please note: I'm not saying, or even advocating, that this is a good thing, or results in ideal outcomes when I say "benign". I'm just using it in the sense that this likely wasn't a malicious decision, but probably one motivated by money at some point, by people who didn't necessarily see the harm in it.