I think it has something to do with the sheer cost of replacement windows and if I fuck up and break one I eat the cost. If the professional installers break it, they eat it.
I have actually replaced smaller windows, like basement windows, but big bay windows I'll pay someone to do that. One advantage I have is that I'm friends with a number of tradesmen and for things like electrical and plumbing I invite them over to inspect my work before calling it complete.
Very understandable.