I think if you actually held both at the same time, you would understand without having to ask.
The discount stuff might get you through a summer with some kids in a pool or maybe a weekend on a vacation.
Good stuff will have nice glass lenses. Won't scratch, defog works better. The strap attachments are typically better. The skirts are often more supple and will conform better.
The discount stuff for in the pool or snorkeling, if it breaks you are not in a place you can't get out of. On SCUBA, you really need a mask that won't let you down.
The "they feel the same" is simply the lack of expertise. I was recently at a friends place in his pool and he had a discount pool mask. It was a quarter the weight. Passed the suck test. Leaked. Lenses were hazy (it was older). I got what I needed it for done (pool light) but I really missed my good mask that felt right and actually worked. Once you know what a good mask is like, you realize the pool masks are a joke. They look like the real thing, and that is what sells them. The look like the bargin, and for those who only play with them as a pool toy they will be just fine. Strap anchor breaks, the surface is only a few feet up.