To the OP -- I really don't see what all the fuss is about. Why do you care what other people use to build their stands, or how they build them? Not everyone is a master carpenter with ultimate structural knowledge like yourself.
Also, you're preaching like this idea of building an aquarium stand out of plywood is some kind of new idea. It's not. Of course it can be done -- how many 2x4 stands do you see in aquarium stores?
As for DIY'ers, those of us who know how to properly join plywood, and are comfortable in doing so, have been building all-plywood stands for years & years. I certainly have been, but I don't degrade those people who can't/don't.
And to avoid running my mouth without backing anything up, like
some people have done in this thread, below are just a few that I have built over the years: