There are sellers who say exactly this in their listings, or that they only ship on thus-and-such a day because of their other commitments.
Fine. Perfectly reasonable. Understood.
It's the ones who just don't bother letting the buyer know what's going on, or who don't respond when contacted after several days go by after the label is created but the item is still not in transit that get annoying after a while.
Sure, you can have a job and a life. But how long does it take to send an email?
This one took less than two minutes.
EDIT: And if you're selling so much stuff you don't "have time" to keep buyers updated, selling IS a job, and needs to be treated as responsibly as a "real" one.
EDIT 2: As Mark implies though, you can't control what other people do. All you can control is how you react.