It's their ego. I'm now at the CTO level, but I still code as a hobby (daily) and will contribute as necessary to a product with coding. A couple years ago, during a stretch with a previous client, I wrote a bit of code. With my second PR, one of the code reviewers tore my code to shreds. It was 90% styling and 10% theory. As for the styling, sure, it was fair enough. I was new to their culture and it was trivial changes -- 5 minutes of work.
The last 10% was simply a theory issue. The reviewer wanted things his way, which would have taken an inordinate amount of time for the feature. I told him that it was unnecessary, and would increase dev time. I was thinking about the entire product process, while he was focusing on how the code should look -- which was purely arbitrary.
He wouldn't waver, and delayed my PR. It eventually was escalated to the head of engineering, who sided with me. It was a trivial feature, and my code was tested and worked -- it just wasn't the reviewer's way of doing things.
Anyway, it got merged as is. But, I can only wonder about how a junior dev may have handled it. Wasting hours of time to conform to a particular senior engineer's arbitrary "way" of doing things -- correctness be damned. As for my own code reviews, I follow a simple process: 1. Can I parse it in 10 seconds? 2. Does it fit the specs? 3. Is it tested? 4. Is it extensible?