In terms of in-house custom engines versus third-party engines (like unity), what has changed today from decades ago, and why?

In the past few years, we have seen an increase in popularity of 3rd party game engines being used instead of in-house ones. Unity, CryEngine and UnrealEngine, for instance, have become more easily accessible now more than ever, with most of them being open-source. The point that they are free will lead to a great increase of development of these engines and allow their whole communities to benefit.