It's because when you build a SPA you give up a bunch of stuff that comes with the traditional web page model, and it's stuff that most teams overlook until it's too late. They are usually fixated on a certain set of performance issues that drove them to do an SPA in the first place, their energy goes into that, and then they kind of look around at all the other stuff and go, oh, we don't have time for those things.

This is why I typically fight SPAs every step of the way with our clients, if any other architecture will work I see if there is a good case to be made for it, because I know that SPAs are usually going to end up shipping broken in some way.