The reason why is that by law (and its fuzzy) people have to have enough to show that they can't accidentally accept terms, usually when payment or providing personal information is involved. So if I sign up for a newsletter on Kickstarter, no big deal. But if I agree to pay $50 for a Garfield poster...a bit more consent is needed.
The reason it's fuzzy is because it isn't a law at all. According to the law, it has to be reasonable that a user/customer willfully and knowingly agree to terms and/or accept payment. That's why receipts are important (show that the information was accepted by both parties and, the second party, aka not the user, is in good faith sharing that information with the first party).
It also means that yes, drop off rates are the highest at that point. And for good reason: that's you at the checkout line, finally ready to pony up the cash. Back down now and forgo the buy, or push on and pay.
It's not legally a requirement because it's an issue of good faith. And, for that reason alone, it's best to force users to agree to certain things for their consent, even if it means they may decide against it. You may lose some users/customers, but you're also doing right by a larger number of them. And doing right by your business