You don't encourage wealthy aliens to hire you by openly backstabbing somebody else who hired you.
In The Nagus, Zek shows awareness that their opportunity for profit in the AQ has been curtailed by their reputation for dishonesty. I think they're very aware that their attitudes are offensive to aliens.
The Ferengi think of themselves as rational businessmen but they are not all that rational, opting for short term rewards with higher risk over long term rewards with less risk.
Ferengi are naturally risk-seeking. They're terrified when confronted directly with physical violence, but they will run straight into situations that risk them physical harm if it means a greater chance at profit. They might behave that way and milk their clients for extra hours just because it's their natural inclination. Or if it's not that, it's the culture. Just like human men have to prove themselves manly by taking stupid physical risks, ferengi men have to prove themselves manly by taking stupid financial risks. Screwing aliens over is the Ferengi version of cliff diving.
