I didn't make the excuse that it is "easier for the developers"; I stated that most people deploying Web applications don't feel the need to integrate those applications with other authentication solutions, and so they will use whatever is provided. The provided functionality could probably be better, but I imagine that there are more people wagging their fingers than working to improve the situation amongst the many different solutions deployed on the Internet today.
As for integration with identity providers, apart from leaning heavily on Google and Facebook, for it to be commonplace the various solutions would have to be a lot easier to deploy - particularly under the conditions currently experienced by those deploying Web applications - than they currently are. Even the commercial solutions can be dreadful: on one project I was involved in, the large vendor concerned had to get consultants in from abroad because there was no-one in the entire country who could offer the necessary assistance.
With things as they are, I think it is somewhat optimistic to expect people - those deploying applications, of course - to enthusiastically abandon the simplest thing that works for them and somehow hook up their application to an inscrutable stack of authentication components with varying levels of documentation for a technology that, if not properly digested, will have people wagging their fingers once again that the resulting solution is insecure because some aspect of the configuration was not fully understood.
All this reminds me of people trying to fathom why organisations buy monolithic solutions (often not very nice and even rather maintenance-intensive) from vendors instead of piecing together freely available tools. No amount of pointing to "how to" documents and calling people stupid is going to change people's behaviour in situations like that.