Vetting source code is a lot of work. You have to be sure all the code you release actually is yours. There cannot be third party code with inappropriate licensing in what you release (say you licensed some code decades ago, and it got copy-paste-edited all over your source code, or somebody copy-pasted a few lines out of some book). Third party code with appropriate licensing may require acknowledgment.
It also can be a lot of work to disentangle the code you want to open up from code you want to keep proprietary.
The commercial side of companies rarely finds such work worth it to, at best, get some good vibes back from a (relatively) small group of persons. I think they’re right.
Also, almost all code is ugly on the inside. Companies may not want to release code as is.