Most monsters in DnD follow the monster stat block rules, not the player rules. So they tend to attack more times than us.
As for the point about Melee characters:
I think that comes a bit down to player expectation of the classes, and the build. Some of it has to do with the changes to the DnD 5e rules that Larian made, but I think that goes both ways.
What's the Point of a fighter? I assume you mean melee fighter? Because a fighter is considered a very good ranged attacker in 5e.
The fighter, right now in BG3, is a two handed weapon power house. Pick up Great Weapon Figher at lvl 4, and consider carefully when to use it (It's a toggled ability). The fighter in my multi player playthrough wreacked face!
Battle Master's abilities also adds a ton of damage.
All in all the Figher did the most consistent damage in our play though.
Rogue: The rogue needs some work (I just made a post about that). But Thief seems to be a very good melee figher too. Getting an aditional bonus action means they can go into sneak, attack with sneak attack and also get their bonus action off-hand attack off. Select the Dual Wielder Feat at lvl4 and dual wield rapiers for lots of damage.
(But yeah Rogue feels very half-done)
There isn't any "tanks" in DnD 5e. The whole "holy trinity" build thing isn't really a thing. Fighers and Rogues are consistent damage dealers, when used tactically correct. The casters tend to be better at battlefield control than damage, with the exception of Warlock who has a decent cantrip in Eldrich Blast.
