I think that's a reason, but not the reason.

There have been times where I've advocated shift left because the people to the right were less expert on their own topics than the people on the left. So why not just cut out the downstream churn.

For roles like QA, Ops, Infosec I've found that the good ones are so much harder to find than good developers. And when you find those good ones, they're worth their weight in gold.

But because they, especially QA & ops, tend to be viewed as lower on the hierarchy they often have lesser talent filling the ranks. Sometimes the entire group will be like this. And at that point, why even bother?