But that's not an efficiency an FPGA provides in the first place.

Additionally, it's not clear how well large mask roms scale. For instance the Nintendo switch cartridges were expected to be mask roms, but instead are Macronix's XtraRom technology, which is essentially a flash cell array made denser by removing the erase functionality. So basically the die gets manufactured with all bits at the same state, a late manufacturing step either empties or fills the floating gate of the bits you want different, and then it's treated as pretty close to a mask rom. It's not even clear if the bits can be changed without a bare die, a floating probe array, and specialized hardware. Though, like flash the electrons in the floating gates will eventually tunnel and cause the data to bitrot.

So from that it appears that even at the tens of millions of chips volumes that would make sense for essentially whatever size of maskrom, the memory manufacturers tap out at 128megabit for a mask rom chip, and push you towards something flash esque.

And at the end of the day, flash without the erase functionality is pretty damn close to a mask ROM, and lets you write it near the end of manufacturing rather than at the something close to the metal 1 layer.