Yes but poloniex return are much lower then casino.
It's risk:reward. You can earn much higher returns in a ponzi, but ultimately lose your entire investment in a scam. Dealing with a high degree of fraud in everything bitcoin, perhaps people are more comfortable with the lower returns of a more reputable investment than all these casinos that are riskier to send your coins to.
Poloniex can also run away with btc or get hacked. Some casino have been around longer then poloniex. Bitfinex was very "trusted" and it was 3% rate a year on lending out. I think to many people underestimate risk on exchanges.
Yeah and at a moment everything seems to be fine with Poloniex, even I am using that site since last 2 years but when it comes to bitcoin we cannot afford to trust anyone.