Who cares when it's done? Often there are external deadlines like a start of production of a car for example which can not realistically be moved because too much planning would need to be redone and too much disruption would cause so much overhead that it could break a company.
Also when price and time are fixed, there is only one thing left for negotiation and it is the amount of features being delivered. Sometimes people have a hard time to cope with it but if you do it correctly it's a great thing for everyone. The most important features could be delivered early and the closer to the deadline the non critical features can be implemented which are nice to have but won't break the product if they're not in place. That's a great way to bring as much value as possible without breaking anyone.
Also when price and time are fixed, there is only one thing left for negotiation and it is the amount of features being delivered. Sometimes people have a hard time to cope with it but if you do it correctly it's a great thing for everyone. The most important features could be delivered early and the closer to the deadline the non critical features can be implemented which are nice to have but won't break the product if they're not in place. That's a great way to bring as much value as possible without breaking anyone.