Code readability has nothing to do with it.

I suppose it could be quantified by the amount of financial damage to businesses. We can start with high-profile incidents like the CrowdStrike one that we actually know about.

But I'm merely speaking as a user. Bugs are a daily occurrence in operating systems, games, web sites, and, increasingly, "smart" appliances. This is also more noticeable since software is everywhere these days compared to a decade or two ago, but based on averages alone, there's far more buggy software out there than robust and stable software.