Your cool for a while until its the brand that your dorky older sister used wear and then you just become another brand. A similar analogy can be made for Facebook, it was cool in 2008, in 2014 when your mom and grandma want to friend you, not so much. Atleast tech companies like Facebook stand a chance of keeping their users as they age.
Not many 30 year olds have both the body and the mentality to wear the American Apparel clothes that they bought when they were in university.
Restaurants are very similar, which is why they tend to start to group together under uber companies that run each brand as stand alone entities. Fashion also tends to follow this playbook. This atleast lets companies diversify their risk among multiple brands.
Its been said that tech companies need to reinvent themselves every 15 years to stay relevant, in fashion, its more like every 4 years and that's depressingly quick.