The line of thought was probably that they may reach a scale that can overcome these overheads since they don't have to pay rent or hire full time staff for every outlet.
It makes sense if you consider very early examples like Amazon which took more than 10 years to become profitable. Unicorns like Twitter and Dropbox are not profitable and are unlikely to become so in the near future, but they have reached a scale that their finances are relatively secure for now.
