Cheap used suspension bikes are bought by the same sort of people who buy cheap new suspension bikes - those who want the max features for their money rather than the max simplicity and reliability for their money. I'm more than happy for idiots to snap up these sort of BSO's - it just means the better secondhand bikes without suspension have less buyers chasing them so it keeps the used prices down. I like being unfashionable in my bike preferences
I'm a mechanical engineer by training, so know a bit about these things.
Something that's that complicated and that cheap, well go figure, something's got to give - in this case it's weight, build quality and, as a consequence, reliability. A reasonably-specced full susser MTB that will actually *DO* the job it's designed for will cost a factor of ten more than the cheap BSO.
For some folk it's all about show rather than go.
(And then you wonder why they give up on cycling so easily.)
Cheap full susser MTBs are the Hyacinth Bucket of bikes.