>>216357369 (OP)
The OG Latin alphabet doesn't distinguish between U and V. That works fine for Latin but not for the Germanic languages, so they used a digraph "VV" for that. And since the name of the letter was "u", the digraph got called "double u" or some continental equivalent.
Eventually the digraph evolved into a ligature, and nowadays you're supposed to pretend it's a letter. And the name was stuck.
Side note the native name in Portuguese is "vê duplo" (double V), although "dáblio" (Anglicism) is probably more common.