Most people can be suitably fit to an off the shelf bike, so there's not really much benefit in that aspect. Then there's instant (more or less) gratification. Weight/$ might be a factor, too - most custom builders work with metal and will be heavier than what could be bought with the same amount of money with CF. Likewise, some may prefer the aesthetics possible with CF as opposed to steel/Ti. Etc. Etc.

​​​​​​Whether or not you agree with another person's priorities, it shouldn't be too hard to imagine why someone would lean one way or the other.