There is a difference between sunny beaches looking goofy cartoony, and just having a beautiful beach setting.
Larian does not know how to create a nice looking setting, everything has to be cartoony and over the top.
Just look at the number of colors in screen in BG3 demo - open areas. There are literary at least +20 colors on the screen the whole time, with high contrasts.
Original BG was much more toned down in its color palete.
I dunno what beaches look like where you're from, but on the East coast they can be quite vibrant with contrasting colors.
As to BG having a toned down color palate or even just being darker in general: That's because it's a product of the time and engine it was released. You're talking about a game released in 1998 (the same time as Fallout 1 and 2).
I don't mind the game being dark and gritty in some portions, in fact, in battle torn areas it should be, but the game shouldn't be held back by engine and technological limitations of the 1990s. Is there some in universe, lore reason as to why the games were dark? If not, then there's literally nothing stopping the scenery from looking different.