Say after 10 months an unavoidable bill of $4k shows up and I got to pay it. If I'm saving up I can redirect that money. It sucks that I don't get the camera when I planned to but otherwise I'm fine.
Under a BNPL plan I have no extra money to allocate and might have to dip into Credit (potentially with high interest).
In short: Savings are fungible, payments are not.
Studies show that huge percentage of the population doesn't have any savings. For them it's particularly dangerous to have more recurring bills.