This reminds me of how when I go shopping for eggs in the supermarket, they are sold in sizes of Large, X-Large, and Jumbo. Nobody wants to face the humiliation of buying a carton of small eggs.

Bras has to fit fairly snug to function correctly and that requires some level of precision.

Eh, it can be argued that the "function" of a bra is to allow women to adhere to the beauty standards set for them by men.

That being said, they still aren't made that precisely, cup sizes are kinda all over the place depending on brand, and even the band sizes (which are kinda sorta close to the actual inch measurement of your ribcage, but not quite) are variable. That's why bras have adjustable straps everywhere, and why if you haven't been professionally fitted for one, your bras probably don't fit you right.

Also that would be expecting a level of engagement with clothing that most men simply don't have.

Men's underwear does have sizes for the dick pouch and they are small medium and large.