Yes it's for Briar and indeed security and reassuring our users there are not backdoors are the important points. For Android we provide reproducible builds and this is a goal for desktop releases too. I'm not sure this is possible for all the platforms, but where it's possible we would like to achieve that.
Apart from that I think there is this fundamental question of whether we want to make ourselves dependent on a closed-source commercial tool. I think the arguments are the classical ones in every debate about open source vs. closed source like:
• ability to improve or extend the software without having to rely on the company behind the software
• risk of loosing access to the software should the business model not work out for the company
• etc.
Maybe you're right with packaging tools being too much effort to develop as a side project, but then again on the other hand wouldn't e.g. operating system fall in a similar category? 😉
I know it doesn't always work out that way, but the way I see it, one of the strengths of open source is actually that multiple people from various projects and companies can contribute to the same shared cause and make a task that seems to big for a single individual feasible as a joint effort. I'm not sure what the exact situation is with electron, but it seems that we can partially build on top of some of the building blocks they already established, such as modifying exe properties just as one example:
https://github.com/electron/rcedit