Well, because they (and their entire organization) may be unable to get a publicly routable IPv4 address, because NCC can't and won't assign any more. You can work around it for now, but in the future this will be a more and more common situation.
It means they can’t get one for free, not that they can’t get one at all. IPv4 addresses are trading for around 20/each when bought in blocks of 256. They won’t be prohibitively expensive for any serious hosting provider for the foreseeable future.
For hobby projects hosted on a Raspberry Pi in someone’s bedroom, I can imagine a static IPv4 being prohibitively expensive, but that’s about it.
