Why does the woman refuse to go the party?

M: The Students’ English Club is having a party on Saturday night. Would you like to come?
W: I would like to, but I have to work at a restaurant on the weekend.

W: Look, it’s already 8. Tom said he’d be here by 7.
M: Yes, but you know what the traffic is like at this time of the evening.
W: He said he’d try not to be late. Why does he always do this? I’ve had enough this time.