Unfortunately, the problem with cowboy coding is that it takes one idiot in the team to ruin it for everyone. As company grows, there are more and more idiots by pure chance, which means you need bigger and bigger walls to contain the blast radius. If you have a team of trustworrthy engineers then cowboy coding is extremely efficient, but it simply doesn't scale, especially considering how difficult it is to evaluate the quality of given candidate when hiring.

I believe that cowboy coding might still be practiced in small companies, or in small corporate pockets, where the number of engineers doesn't need to scale.