Consider a large corporation goes on strike, like a Boeing or Raytheon or a public utility, that is considered "essential" to public safety or national security. If the union isn't at least somewhat reasonable, it will cause the business to risk going under, or the govt to nationalize its property, or them to succumb to competition. It's a form of mutually assured destruction. This is exactly what happened to the automotive unions after the 2008 financial crises. They re-negotiated existing contracts to ensure the viability of the business. Many hourly rates were slashed to less than half of what they were just a year prior.

Contrast that to something like a police union. What is the backstop to prevent union demands from getting unreasonable? Essentially very little. We can't just stop having a police force. There's no competition to go hire the next contract with better terms. If the union knows they can't go under, their demands can get more extreme. Plus, they work with a captured market. A private business has to essentially woo customers to remain viable, public organizations have established clientele that have no other options and limited oversight compared to regulated monopolies in the private sector. It's also already been said multiple times that police are the very mechanism often used to break up strikes, so who prevents an illegal police strike? They are also well equipped so the threat of sanctioned violence by something like the national guard is not a good option. None of these issues are generalizable to private organizations of large scale. There's just less mitigation to keep public unions reasonable because public services are different in their application.