I bet for most of these conferences, those costs are rounding errors in the grand scheme. When Microsoft has Microsoft build, the cost of the space and food is nothing compared to the return they are going to get from companies who turn around a pour money into azure because of what they demonstrated at the conference. Same with other companies.

The cases where this probably isn't true are going to be the conferences like Debconf or BSDConf. Mind you, these conferences have been hosting all their sessions on youtube for years.