It’s really very ironic that you’d mention WhatsApp and Zoom in this context, the latter in particular.
Zoom does not support E2EE (and lied about their security capabilities), leading to big problems with people intercepting or joining supposedly private voice calls. It’s so much of an issue, that it even has it’s own Wikipedia page;
They’re still working on preventing that now.
WhatsApp is less vulnerable, but it has a very small maximum group size, and a maximum call size of 4 people. That’s how they can use E2EE encryption. Discord on the other hand doesn’t claim to be secure. It’s a public social media / chat platform that happens to allow private messaging and voice calls, just like many other social platforms.
Discord and WhatsApp are not equivalents. Discord is a social group application, hence allowing servers with populations three full orders of magnitude greater than WhatsApp (I believe the largest Discord server has nearly half a million users), and allowing voice calls between 25 users and video streams to 50 users.