People have learned that something might look easily replicable, but end up having unexpected nastiness. Further, replication takes time, and requires assigning people to the task.

In my company we often buy-in technology because, even though we could write it ourselves, and might do a better job in the end, we are pretty sure that there will be hiccoughs and unexpected difficulties, and during that time we will be dedicating manpower that could be used doing things that other people can't replicate.

dl;dr: Risk and time.