@foundinblank unfortunately, I would estimate that most of our webinars have non-stop talking that builds upon the content on the screen (or at least explains it). I know that we’ve tried several video services—I think we are currently using wistia—but youtube has not been one of them. On our end, we benefit from viewer analytics that youtube does not provide.
I would really like to have transcripts, certainly for the hearing impaired but also for native speakers of other languages. So why don’t we? If there is one thing I’d like to convey about the webinars, it is that they are a lean and free operation (this goes to @kmprioli’s comment too). We usually make a webinar happen by dragging a developer away from his or her work (and, in some cases, comfort zone!), and the webinars do not directly make money that can be reinvested in the webinars. I think you can expect to see future refinements (including transcripts for existing videos), but for now things are lean and informal to get information out as quickly as possible.