Answer: Professionalism. An editor or publisher will only ever get one chance to “steal” someone’s work. After that they’re out of business, sued into oblivion, and Chernobyl when it comes to ever being employable ever again. The only writer who is worried about an editor or publisher stealing their manuscript is the noob who doesn’t understand that their work isn’t worth stealing. The amateur. The wannabe too scared to break in who is looking for a reason – any reason – to ensure they remain anonymous and unpublished.
Unfortunately, you’re a poster child for why it’s so difficult to get manuscripts in front of agents and studios; you say you’ve been ‘ripped off’ which is a HUGE red flag and wave-off. Nobody wants to deal with a problem that could lead to a lawsuit.