I used to buy cheapest I can get away of everything and I have to tell you, there is real difference between cheapest bikes you reference, bikes that cost few hundred $ and $1000 bike.

The difference is in speed, effectivity (how tired you get per km), comfort, how much it hurts when terrain is bad and pretty much any other factor you can think off. Those cheap bikes are fine if you go to work and back, 15min drive each way. Or for kids to play around town.

But if you bike a lot, then you really want better bike.