The blank ones are made by children who are sold into bonded slavery by their poverty stricken parents. They are then kept in darkened rooms with their feet nailed to the floor so they can't escape and are forced to work 23 hours a day.
It's not all bad news though, they do get a half days holiday every alternate leap year.
To be serious though, by not buying clothes with labels or slogans you avoid the fashion trap. Fashion victims are called that because they fear the ridicule of their peers if they are seen wearing last weeks/months/seasons fashion.
Those of us who choose not to wear branded clothes can buy a shirt or whatever and keep it until it wears out. Huge amounts of the planets limited resources are wasted by people who throw out stuff before it is worn out just so they can keep up with fashion trends.
People could also choose to buy clothes made to high ethical and ecological standards, although they often look like they were made by colour-blind hippies.