What else are you going to make them out of?
Plastic or resin would have to be thick or well engineered to not sag when the tank was lifted with some water in it. It could work OK for very small ones, but larger tanks it would be difficult. You would need support for plastic all along the bottom, like a table, not like some stands that just support the edges of a tank.
Metal is possible, but you would probably want it coated in epoxy or some such to make it unaffected by water. You could use stainless steel, but piece that is as strong and stiff as an equal size piece of tempered glass would be heavy and more expensive.
Plus, I wonder about different rates of thermal expansion for different materials.
Probably a metal reinforced resin would work, but again, it would take more engineering.
And finally, an aquarium maker is set up to work with glass; they have the tools and knowledge to make it work. And it does work, all the way up to the biggest commercially available sizes. It may seem like a possible weakness, but the bottom is not where most tanks fail. (It is the seams or from the front/back bowing if supports are removed.)