You’ll have to ask the people who started calling them milks hundreds of years ago.

There are plenty of plant sources of Omega 3. Flax seeds, walnuts, soybeans, and canola oil all have decent amounts of omega 3 in them. As for protein, legumes generally have a bunch.

Really, the only thing a vegan needs to supplement is B12, but even that gets added to a bunch of stuff like breakfast cereals and plant milks if you consume those.