Why? Interested in your reasons. One plausible one is that trusting an author you know can be more practical than examining every line of a massive open source code base. Could that be a reason? Others?

Shotgun blast: open source software is easier to read. Is the quality higher? Depends: there are some terrible vendors, and there are some very shoddy open source security projects. Open source software, with a couple of exceptions, are rarely audited professionally. Widely used open source projects get shaken out for memory corruption and XSS. They do not as a rule get thoroughly evaluated for cryptographic flaws.

A more precise way to state my preferences:

I trust _crypto_ from Microsoft, Apple, or (especially) Google more than I trust _crypto_ from a developer I've never heard of before. I do not as a rule trust rando closed-source projects.