Others have covered the details of labor laws in the US, so I won't touch on that, but your question does make me think about why those kinds of labor protection laws are even seen as a necessity. And I think the answer to that is we (most people, not just Americans) view jobs as equal to livelihood.
But it makes you wonder what the world could be like if we had a universal basic income, where getting fired wasn't actually the worst thing that could happen to you. It might still suck, but you'd still be able to have a roof over your head and food on your table while you searched for new work. This, critically, would give you more negotiating power when finding new jobs, as you'd likely be less desperate for a job, meaning you could credibly insist upon better pay and better conditions.
But we could take this one step further. In economics, there's this concept called an externality, which is when you do something that affects someone else as a side effect. When you do something that harms someone else as a side effect (e.g., pollution), that's called a negative externality. Negative externalities are actually a major problem in completely unregulated economies, because they cause the "invisible hand" of the free market to fail to achieve optimal distribution of goods, i.e., a market failure. The classic example of this is carbon emissions -- the true cost to society of carbon emissions (from climate change) is not reflected in the cost of providing carbon-intensive goods, thus we have a tendency to over-produce and over-consume carbon-intensive goods and services. That is, the economy would be better off in the long-run if we emitted less carbon than we currently are, despite the short-term profits of polluting. Anyhoo, this mismatch between sticker price and true cost to society is why carbon tax is almost universally regarded to be the single best climate policy: by accounting for the costs of the negative externality, you can fix the market failure, and the invisible hand can once again work as it's supposed to.
But where this relates to where I was going is there are also positive externalities, where you have a positive impact on someone else as a side effect of your activities. An example might be doing regenerative agriculture or rewilding a patch of land -- the pollinator habitat you provide or the carbon you sequester has positive impacts on other people. And like how negative externalities tend to lead to overconsumption, positive externalities tend to lead to underconsumption. I.e., the economy would be net better off of more people did rewilding and regenerative agriculture, despite the short-term immediate costs they incur. And much like taxing negative externalities (e.g., carbon emissions) is a good way to correct those issues, subsidizing positive externalities is a good way to fix the issues of insufficient good activities.
So imagine if we not only had a UBI, but if the government also would pay you to plant trees or develop/maintain open-source software or any number of other activities that produce positive externalities. If we had these alternative means of maintaining a basic level of livelihood, then maybe we could decouple existing from jobs, and we wouldn't feel a strong need to coerce businesses into holding onto people, nor would we need to coerce them into paying people enough or giving good enough working conditions -- companies would have to pay well and offer good conditions and not fire for unfair reasons, else they'd struggle to fill vacancies.
We all saw how companies begrudgingly had to pay more during the "great resignation". Or look how the professional class (e.g., doctors, engineers) get good pay and good conditions, precisely because they're hard to replace. Give workers more options, make them less desperate, and they'll be empowered to negotiate better pay and better conditions for themselves. Sure, some regulations would still be necessary, but I think there's a lot of elegance in a bottom-up approach to labor relations.