The people who are invested in their home-grown solutions are usually pretty bad at doing tech support for them. They enjoy writing code, and it doesn't take long before you've created enough surface area that you couldn't possibly keep up with requests even if you wanted to.
Which they don't, because nothing has convinced me of the crappiness of some of my code more thoroughly than watching other people try and fail to use it. They are a mirror and people don't always like what they see in it. That guy who keeps talking trash about how stupid everyone else is for not understanding his beautiful code is deflecting. If they're right then my code does not have value/isn't smart which means I don't have value/am not smart, so clearly the problem is that I'm surrounded by assholes.
I don't think we would be using as much 3rd party software without StackOverflow, and almost nobody creates a StackOverflow for internal tools. Nobody writes books for internal tools. Nobody writes competitors for internal tools which either prove or disprove the utility of the original tool. All of those options are how some people learn, and in some cases how they debug. You're cutting off your own nose when you reinvent something and don't invest the time to do all of these other things. If I lose someone important or get a big contract I didn't think I could win, I can't go out and hire anyone with 3 years experience in your internal tool, trading money for time. I have to hire noobs and sweat it out while we find out if they will ever learn the internal tools or not.
The older I get the more I see NIH people as working a protection racket, because it amplifies the 'value' of tenure at the company. The externalities created really only affect new team members, delaying the date when you can no longer dismiss their ideas out of hand. You have in effect created an oligarchy, whereas most of the rest of us prefer a representative republic (actual democracy is too many meetings).