It's likely because they actually use funds deposited to their site to invest in miners, but don't return significant amounts of earnings to their customers. Just because a cloudmining company claims to own hashrate doesn't mean they're legitimate.
This one might not be an outright scam, but it's still doubtable if you'll ever get ROI with them.
I agree with your statement. From what I have read in the Bitclub thread linked
here in February, one user's gotten a 9.6% return rate in seven months, and that's likely subject to change with difficulty adjustments- certainly not good if you ever want to see all of your money again without having to lure more people into this scam. The entire thing's really just a crypto MLM. Just more proof that cloudmining's a terrible idea...