With any regular bank or brokerage, you can take your money out whenever you want, on fairly short notice. This applies even if you have tons of money in it. Now, if you have a lot, like lets say multiple billions of foreign exchange reserves, then placing a sell order on all of it will drop the value, the price will have to go down for all of it to sell, but you can do that, if you wish.
Heck that was part of the problem in the big downturn a few years ago. People were panicking and selling their whole portfolio at reduced prices, which of course feeds back on itself. A guy I know is a financial manager and he would try as hard as he could to convince people not to, since it would realize big losses for them, but they wanted none of that, they wanted it in cash (or bonds, or other safer stuff) and they wanted it NOW. So, he did as he had to and followed their wishes.
As the parent points out, the reason Bitcoin wouldn't let you is ponzi type reasons. If someone big cashes out all at once, that could cause the value to drop a lot, which could cause the whole thing to tumble down. They are trying to make sure that doesn't happen, to prop up the farce.