Indeed, but maintaining software that is a core part of your business is a very different game than supporting (complex) internal tooling. They don't bring reve

Indeed, but maintaining software that is a core part of your business is a very different game than supporting (complex) internal tooling. They don't bring revenue, are a time/money sink and not many companies can spare the resources to do it properly. Anedoctal experience: a company I used to work for had all the core stack developed inhouse: programming language (yeah), compiler, database, etc. No one worked on it apart from the founder and a couple of other engineer that were no longer there. A few years down the line a big client asked if we were able to integrate our software with their PLC and this had to be developed from scratch, as it wasn't in the scope of the programming language. The quote sent to them was in the six figures and up to 2 years for it to be ready. Every major programming language has a library that does this. Eventually the company lost the client.