Because if employees aren’t tied to their job because of health care, they have leverage in things like salary and other benefits like vacation and company perks. You don’t like what your company is paying? Leave. Don’t get enough time off? Leave. Want to work from home and they won’t let you? Leave. Etc.
In the current system, many people who are reliant on their health insurance to survive don’t have that choice. Hence, businesses can get away with lower salaries, less vacation time, etc.
Edit: I should also note that small businesses probably fight for free health care because obviously it’s the opposite situation: it’s much easier to keep employees when you can actually afford them and don’t have to pay for health insurance.