In the worst cases you get people who just want to say they made an OS by slapping their name on an existing distro after changing the default background image or making a couple tweaks. That's a lot easier to than to contribute something meaningful to an existing project.
More charitably, it's faster and a lot less complicated to modify a distro to your liking than trying to get a major distribution to cater to your whims and philosophy and making your version avilable to others gives them the option of easily installing your new features/modifications as is or using what parts they want. That's the joy of of FOSS. Even when the changes are modest, if they are shared and even a little useful then someone else can incorporate them or build off of them.
More charitably, it's faster and a lot less complicated to modify a distro to your liking than trying to get a major distribution to cater to your whims and philosophy and making your version avilable to others gives them the option of easily installing your new features/modifications as is or using what parts they want. That's the joy of of FOSS. Even when the changes are modest, if they are shared and even a little useful then someone else can incorporate them or build off of them.