In BG3 it is a balance mechanic. Heavy objects tend to be completely OP and are used to cheese combat. encumberance limits this and even allows building your character specifically for this playstyle.
In Bethesda games encumberance is in part there to protect players from themselves. If every object can be picked up (and that is a design principle in those games) and every object has a Value, then the optimal strategy is always to grab every single object you can find and then sell everything at once. If that does not sound like fun to you that is because it is not, but still i know multiple people who play those games this way even with encumberance in place. Players will always find a way to ruin their own fun, the only hing you can do is to put systems in place that disincentivise these behaviors.