Is it the screen that’s cracked, or the glass panel that covers the LCD display?
It’s far more common for the glass to be cracked, because the glass is exposed. It’s possible, but less common for the LCD to be cracked, but since it’s behind the glass, it’s (somewhat) protected from damage.
I suppose it’s possible for both to be cracked, if something was thrown at your iMac with sufficient force (or if your iMac were lying on it’s back, screen up, and something heavy were dropped on it).
But when I’ve serviced iMacs with “cracked screens,” it was always just the glass that was cracked. I’ve never had to replace an iMac LCD display.