All paper sizes are based on historical plate sizes, just like film sizes. But while film has "shrunk" a few mm in each direction to fit in film adapters in plate holders, paper is still the full nominal size.
That's why the standard sizes are 5x7", 8x10" and 11x14" in UK and USA; and 13x18cm, 18x24cm, 24x30cm and 30x40cm in (much of) Europe.
There's a lot of equipment (trays, tanks, easels, frames, portfolio books, mattes etc) that is standardised along these same measurements. Making "off-size" papers would perhaps satisfy some, but it would p*ss off a lot of others.
I'm p*ssed off enough already that the manufacturers can't be trusted to make the 24x30cm papers the correct size - most of them mark it 24x30cm, but it's really 9.5x12". Which doesn't fit in standard frames, sleeves, tanks, or my camera.
