So, apparently universals don't have existential import because you'd have to check each and every particular member of a class denoted by a term to know whatever is claimed is true for all of them, everywhere.

Saying that, a term also connotes attributes / qualities that must be shared by every particular member of a class in order to be grouped and denoted by it in the first place.

For example, why can't a universal categorical proposition like "All Trees are Plants" have existential import?

• The subject and predicate classes are both defined by their attributes, so we know all things denoted by the terms - i.e. trees and plants - must have and share those attributes. Otherwise they wouldn't be trees or plants.
• In this case, a defined attribute of a tree is that it is a plant. Any thing that is a tree must be a plant by definition.