They won’t steal it but they will float it to their networks for validation and if any of them are looking to pick something up and run with it, now they’ve got the same idea from a known-quantity team.
It may happen but the benefits of being secretive are lower than the benefits of sharing an idea and getting constructive feedback. One of the earliest fallacies I learned as an entrepreneur is that ideas need to be protected lest someone "steal" them or "beat me to the punch." It's just not true.
The benefits of sharing an idea and getting feedback far far far outweigh the minuscule risk that sharing an idea will spark a competitor that will end up outcompeting me.
Now - having said that - posting your ideas in detail on a public thread on Hacker News doesn't necessarily have the same benefits, so I can see the hesitation ;)
Yeah there is a level of paranoia that can be problematically isolating.
In my experience it really depends on the type of idea, the other types of validation you already have, and your existing relationship to the funding network.
The more moonshot the idea, the more validation you can get outside of funding pitches, and the less cozy your relationship with the (well-networked) money, the more risk you get from sharing.
Many of these companies will pitch the big-picture what and why but not disclose the how, which is the magic. On the other hand, if the what and why are all the value and you share them before you’ve at least somewhat captured them then you risk giving away the opportunity, because the how is just what you’re asking for: the resources.
It absolutely happens.