Maybe it's about reputation and trust. Valve gave access to the source code of their major games, and unfortunately the code ended up being leaked. Even a huge company like Valve trusted people with their source code, and the people they trusted failed to keep it private.
Another thing to consider - what if someone just "looks" at your finished game, and they make a new project and try to copy your game from scratch, WITHOUT your capx? How would you respond to that? We are game devs, we look at games and we can deconstruct how they work, so it's not impossible. A group of people managed to recreate Super Mario 64 with no source code at all (altho it took a huge amount of time).
On a similar topic - How would you respond to fangames? Fangames are a popular area, and a lot of people get very upset when fangames get banned or closed.
Here's what I would do to try and protect my project that I must send to a company or share:
Upload a video recording of your game, including you editing it in Construct, onto a long-existing website (e.g. YouTube). Keep it hidden/private. This is just to keep as evidence in case anyone tried to steal your work (you can unprivate the video, and it will have a date stamp of the upload date that is impossible to fake).
Keep copies of your saved capx file throughout development. You can then demonstrate the stages of development if you needed to prove this.
If you have a community/fans, and you have a good reputation, then you can bet that your community will defend your ownership online and will leave negative comments on any content that is stolen from you (whether you like it or not, as they adore your game - I am not suggesting that you tell your community to defend you).
Hope any of this is useful!