Is it 2x2TB or 1x4TB, though? Obviously, it'd be cheaper to do 2x2, but there's a decent chance you're actually getting 1x4 on the iMac.
The biggest NVME drive I can find is 2TB. Apple might be offering a 4TB option, but isn't that less good? You lose all your data at once instead vs losing half.
I doubt they are hurting for real estate with that screen size, it might just be better to add another M.2 spot on the board. Then they wouldn't have to push manufacturing limits to source a 4TB drive and can get higher yield 2TB drives.
But not if you consider they have to honor the warranty.
Most SSD manufacturers offer a warranty that is included in the price of the drive. The Samsung 950 PRO warranty is 5 years or 400 terabytes written.
The real difference between Apple and DIY is $300. On their configuration page, Apple's prices are +$800 to go to the 2TB SSD, and +$2800 to go to the 4TB SSD. That's a $2000 price jump to add 2TB. If I did it myself I could add 2TB for $1700. That means that Apple gets all the profit that a retailer would, plus $300.
A $300 premium is still a little gougey from Apple considering it takes them all of five seconds to slap the extra SSD in on the assembly line.