The primary reason is community. You can watch the talks online, even tweet at the speaker with questions -- but there's something special about walking around the event floor and bumping into them for a short conversation (or even sharing drinks at an after party).
Events/conferences are also platforms for speakers. They may not have the same following, and posting the talk on their YouTube or Twitter would just fall flat compared to a network built up to broadcast these topics.
Hearing a talk in person is also much more visceral than any video. Depending on the type of talk, it can be much more moving and motivating. You can also ask questions and engage the speaker (if they allow).
It's all just different points of exposure. In a world where there are a myriad of platforms for content, you have to spread your influence as far as possible to ensure the highest saturation. For example, some people that visit the event may not have the time to follow videos or tweets online as closely, so they use events to catch up with major topics.