> People are used to identifying themselves with a sequence of digits. If you're dealing with the tax office, the water company, the electricity company, or whatever, you get asked for your customer number, your meter number, your reference number, and so on, and usually these consist mostly of digits.
It's not a level comparison though. You don't need those numbers often, and when you do, you have all the resources you need with you, and it's not urgent. You generally need those when you call customer service, which you only do when you're at home and have time to spare. But if every email, messaging/social networking app, bank, credit card company, shopping site, etc. required me to go hunting for a 10+-digit number, I wouldn't have any trouble understanding it, but I would get fed up pretty fast.