I'm looking at building a woodworking bench for myself and have
familiarised myself with the plans that are sold on the internet. All
the Kits and commercally availiable, ready built benches feature fine
lumber of 3+ inch thickness for the top - be it birch, beech or other.

Since I am operating on a smaller budget, I was wondering what reasons
are there why a sheet of plywwod - 3/4" or 1" oak or birch supported by
doug. fir (4x) lumber would not be used?

Would the plywood warp over time? Is that the reason? I'd rather learn
the answer from someone knowlegable now than build it and learn the
hard way.

There must be reasons for the choice of real lumber for these
workbenches I see foir sale (other than they are beautiful).