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.

I agree with Nathanael's statement. Part of developing a community of scholars is networking. While online activities open new opportunities for networking (for instance, cheaply bringing together people from different regions on a shared platform) there are significant limitations to networking only online.

As researchers, we conduct a lot of our work in isolation - even when we collaborate with others, it is often filled with internet exchanges rather than in-person contact. The in-person nature of conferences and guest speakers gives greater opportunity to have high-quality interactions.

I do agree that it would be helpful if universities and organizations provided more of their organized content online, but only in addition to in-person interactions, not in place of them.