I think it is partially that (but minor). Partially embarrassment (frequently code is rushed). Partially people view code/data as a trade secret. The latter I find anti-scientific, especially since a lot of funding comes from public money. I'm okay with holding on to it for a small period of time (because we live in an unfortunate world where sharing knowledge can't come first and people need to secure funding), but I don't think this should be a default mode that people do. Luckily it does seem that many are turning to GitHub/BitBucket/GitLab to make their code available.

And everyone that's in a PhD or has one knows that the majority of work you do is failing (but you learn from that failure).

Side note: I wonder if imposter syndrome would decrease if we were aware about one anothers' failures and didn't only see the accomplishments.