This is an incorrect assumption. E-mail and even SMS text messaging are heavily relied upon.
This very story shows that Amazon execs were using Signal, not some custom application.
If a company was found to have developed a custom in-house app that was exclusively used by executives and their friends, it would immediately become the central target of their investigation.
> Why would they risk using a popular platform?
I think the part you've missed is that if someone is discussing something sensitive or illegal, they avoid writing it down in any medium at all. They won't develop an in-house app for messaging because they're not going to be messaging these things if it can be avoided. Instead, they send a meeting invite and then discuss it in an ephemeral medium like voice or in person.
> I also assume this is how most illegal collusion is done as well,
Smart people colluding for illegal activities aren't going to take notes or leave anything in writing. It's done in person or in voice, not via e-mail or in-house apps.
err- i dont mean to be rude but, what gave you that assumption?
Meta has Workplace, for example.