Because you have multiple employees, most of whom only have one job, and who, unlike the employer, depend on the relationship for their livelihood.
I'm not sure I'd use the word "professionalism" to describe this obligation. People have given me notice via email before, and I don't think they were unprofessional for it.
Professionalism isn't like pregnancy, it's not a binary state. I would categorize a person who resigns by email as an act that's below average on the scale of professionalism.
I mean seriously, how hard is it to present your resignation letter in person or by telephone?
In any case, let's agree to disagree, since everyone has different standards.
