"I'm a pampered software developer. I can't believe how much I complain about my job, despite the good wages, free snacks & beverages, flexible work hours, telecommute option, and having the boss refer to me as a rock star."
Define "come to work". Am I considered at work if I'm going through that annoying bug during my morning shower, or spending my weekends networking online with technical colleagues?
I doubt this is a good comeback as the other person might very well also be thinking about some company related stuff and talking to colleagues on weekends.
There really isn't a whole lot you can say; quips certainly won't help.
The problem here is a fundamental misunderstanding by your new coworker of exactly how the employer/employee relationship works, as opposed to how he, or really most people, believe it works.
In his understanding, the relationship stops with the employer, and the employer is paying for his time the same way he might pay at a meter for parking his car. What he's doing while his car is parked will vary, but he is expected to pay the same amount at the meter per unit time regardless of how worthwhile being parked there is to him at any point in time. In the same way, an employee is paid for seat time. The service he provides to his employer may vary depending on what the employer has him doing, but he expects the same remuneration regardless.
I suspect by your confusion, you see the same misunderstanding as I do. The relationship chain does not stop with his employer; instead it recurses out one or more levels from there until it reaches a customer paying for the product. This is where the value of the work is realized, when you find the person who will purchase the good or service on behalf of themselves, rather than on behalf of someone else further up the chain. More often that not, this last person in the chain will end up paying based on something other than time. If this customer refuses to pay more than employer is paying for your time, you are not productive enough, regardless of what sort of sense of entitlement to employment you think your diligence and hard work provide for you. You cost more than you produce; thanks for playing.
If you generate enough value for the actual customer that your employer is happy with the cut they get, then it absolutely should not matter how many hours you put in to generate that value.
It really comes down to a question of which is more important in being an employee: the value you produce for your employers customers; or how well you demonstrate your willingness to work hard, be a team player, demonstrate a strong work ethic, be diligent and obedient to the chain of command, or any of a myraid other things that might increase the value you produce to your customers, but are not the same thing as producing value to your customers.
The market is brutal; it couldn't care less how pious and moral you are.
Answer: "Because the value I provide to the company through the work I do more than justifies my salary, regardless of how many hours I spend, and what time I arrive, in the office."
"So, why do you get to come to work so late?"
What do you say to that?