It’s a 5E thing, it was the same for BG 1 & 2, it is a vastly common Fantasy trope, and once we get Sorcerer they have the monopoly (outside of a feat) on casting a spell quietly.
You're mistaking one thing with another. Sorcerers can cast without saying anything at all. Others need to pronounce incantations for many spells, but there is no requirement to shout them as loud as possible. Such requirements would make some of the spells useless.
