Because the employer is the one giving money to the employee, not the other way around.
"If you get copyright then do you also pay developers overtime for all that work?"
If you're salaried, there is no such thing as overtime. That's what the word "exempt" means on your tax forms. This is also why Google doesn't care when I get in to work at 1 PM, or when I take an hour long lunch break, or when I spend my whole time going to Tech Talks instead of working. As long as I get my work done over the time periods required by the business, it's none of their business how I spend my day.
Employment law recognizes many different categories of employees. If you're a contractor, you're paid for your work - you and the firm agree precisely on the deliverables, they give you a lump sum for the job, and then it's completely up to you how you build those deliverables. If you're hourly (non-exempt), you're paid for your time - you receive a set hourly wage for each hour that you work, and what you do outside of that is your own business. If you're salaried (exempt), you're paid for your ideas - you can manage your time as you please, but anything you think of relating to the company's business belongs to the company.
You should know exactly what you're signing up for when you sign those employment agreements, and pick the form of employment that suits your goals best. But remember that companies adjust the amount of information they share with each class of information accordingly - no contractor is going to know how Google's search infrastructure or ranking algorithm works, but a former employee will.