If you have a special skill set (as most of the posters here do), then the work is never done and there is no one else to easily assign it to.

So even if the boss has slack in his a budget, it usually isn't enough for another employee, and certainly not the same skill/experience/ability of most coders. If he did, then he'd assign different work to them unless the project is deemed a 2 man job. After a while working solo on a project, it CANNOT be handed off to someone else without a training / acclimation period where far less work is getting done.

No matter how good they are.

So the boss will always push for more time spent to hit the deadlines. If he can push salaried employees to do it for free, and isn't a buddy, and the company doesn't have a firm policy against it, he will.