Yeah I hear what people are saying, but at the same time these conferences cost money to run and people pay money to attend, and I wouldn't be surprised if some are paid to talk.
It would be a bit unfair if someone paid money to attend all these speeches/lectures only to find out they could have watched it online for free.
Typically though the videos do end up going online one way or another after the conference is over, just take a bit of time to pass.
That premise is not true.
90%+ if not all of the most expensive/respected conferences in tech are today streamed/recorded and they have been studies made -vast majority of the ones that aim to go - mostly for networking reasons will not stay at home and watch the stream just because they can so.. it's more for people that can't attend for various reasons (outside county, not afford, etc ) and for spreading the word and building the brand for the conference , I can guarantee that if the conference gets recorded and streamed more will attend next year then other way around if they have the opportunity next time.