Food isn't constantly touching plastic :)

And I have some glass ones that do stack, as in they fit into themselves, albeit not as tightly as a plastic set. Or another set that nests a smaller one inside of the bigger one with lids on, then I stack the big ones on top of each other.

Also, they don't change colour as they age or stain depending on what was in them or how much iron was in the water when they were cleaned.