There is the 'only so much time in the day' issue too. If you have millions (or even 1000's if they are high touch) of customers, you'll never be able to code if you're spending your time talking to them - and it will probably be required to develop specialized skills in talking to them too.

At some point, specialization has to happen. It also has drawbacks. Small teams are often dramatically more effective than large teams for similar reasons (ownership of the problem, less communication/education overhead), but they can only scale so far.