Why do conferences and other academic events still mostly happen offline?
Conferences are not organised for the talks alone. They are more like the academic equivalent of a pool party or a weekend football league. Although on the face of it the purpose might look like spending time in the pool or playing football, people really go there to meet other people.

As a PhD student it took me a while to realise this. Most people attend conferences to get face to face time with other people in thier field, and often the talks act as adverts for collaboration. You'll notice that there are often conference dinners and mixers designed precisely for meeting others.

Every time I've heard someone talk in earnest about what they liked about a conference, it's always that they managed to catch up with a colleague or similar.