Large software companies still hire their own accountants and lawyers and HR correct? And this is largely expected and understood to be a requirement. Why should IT be any different?

Once you hit a certain size, it doesn't make a whole lot of sense to say something like "let's outsource our IT, they are distracting me from focusing on our core competencies". You should have a CIO/CTO and a whole bunch of other people to be distracted for you. In fact, that's kind of the purpose of having an IT department, so that you can focus on your core competencies rather than keeping up to speed on the latest changes to AWS services or whatever.

This is weighted of course. The more your industry requires competent IT, the larger a % your company should probably be handling in house because then it is a part of what should be considered your core competencies if you want to stay around for very long.