All the games you cited were sequels and/or expensions that had a previous game that you should/could play before playing that one. BG3 stands on its own.
You are blatantly wrong. By definition, BG THREE is a sequel to BG1 and BG2.
I fully agree with the OP and have made this exact point myself some months ago. The starting point of this game is completely unrealistic. As someone else mentioned in another forum, BG3 is attempting to recreate the player experience of playing BG2, but without BG1 ever having existed. Just as it would've been utterly ridiculous for a bunch of level 1 characters to awaken in Irenicus's dungeon in BG2, so too it is equally ridiculous for a bunch of level 1 characters to awaken on that nautiloid. Furthermore, the character backstories of every single one of the five origin companions is way beyond what would be realistic for a level 1 character.