Those usb sticks are built by grouping memory chips on a pcb.

Imagine how hard the logistics would be if instead of a standard 6 pack beer we had 7 packs, 5 packs, 11 packs, 13 packs

In thus case a 6 pack is a memory module of 8gb, if you ask the manufacturer to build a 42.069 gb usb they will say "i can do either 32 or 64, im not going to design and manufacture a special design just for you (i can do it, but it will cost you more than making a 64 gb version)"

Edited because 40 is a multiple of 8 lmao