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.
I’m really not. According to many many spells in 5E, they have a verbal component. I’m not saying that it is canon to shout like Skyrim. I’m saying it has been this way since BG1.
Furthermore, by RAW, you can’t sneak attack with spells even as an Arcane Trickster. Why? Because of the verbal component (also because WotC).
