[max51]

Because that's how you end up with projects that take 3 years to plan instead of 3 months. A live Q&A where all of the experts who can answer questions and everyone interested in the subject who may have questions are in the same room (live or virtual) is a lot more productive compared to what you are suggesting.

If something they said in the main presentation was missing important details that you need to do you work, why do you need to wait days/weeks for them to gather all the questions, find all the answers, and publish a video, when they could just answer it live in a few seconds?!

[brightlancer]

Having 500+ people on a project is how something takes 3 years to plan.

"At that scale there'll be no interactivity during the meeting anyway."

[max51]

There is interactivity. Each company has their own way of doing this, but it's typical that they have someone reading the chat to gather questions and that higher ranked employee can directly speak to ask questions.