Even Adam Smith knew that 'free market' is idealization and don't fit into everything without tweaking. Most non-regulated markets suffer from market failures and contracts have negative externalities. Smith's invisible hand was not all seeing hand.
As a market advocate myself, I approach markets as an economist. They are something can be made to work, but it usually involves mechanism design (= reverse game theory).
Collective bargaining and labor laws can have both positives and negatives. To make it work well requires discretion. Workers competing against each other with lower benefits has some very negative effects for the society.
It's because the free market is a myth, and what actually exists is all kinds of forces playing against each other, using influence, media, money, laws, and the government for their purposes.
Now, the most powerful forces are those of the people with (m/b)illions in the bank, expensive layers, friends in high places and the ability to move their operations wherever they want, and those are rarely people looking for work -- rather they the people looking for employees.
Which is why laws are required. Because a democratic government represents each person equally -- each has one vote, regardless of their wealth (again in theory, like there's no free market, there's also no actual democratic government). So it serves as a counter-balance between small people and big people/coprs (governments can be in bed with big people/corps but they need to pander to small people too in order get their vote).
Add to that, the fact that without any government at all, you don't get anywhere close to a free market either.
At best you'll get the heavier players doing everything they like -- and having private armies and thugs enabling them, to which regular people can just suck it and play along.
This has been the case in some places in developing countries for example, where, while there nominally a government exists, it's so in the pockets of the local moguls that it's just like their private mercenary enforcement force.
>Some guy might not care about 30 days vacation but might prefer more money, as an example.
Without any kind of legal pressure, guys and gals are gonna get neither "30 days vacation" nor "more money".
You're also really cute. Hmm. Let's see ... maybe I can help you if you help me ...
It's all about the power differential.
Sure, people with a long track record or specific skills may have more room to negotiate, but for entry-level and/or unskilled work you'd be unlikely to get more than the minimum enforced by law.
I have a similar reaction to your comment here. It is as if you know exactly nothing about the long history of labor markets. We started out with very unregulated markets, and have moved away from that for very specific historical reasons. You could reasonably argue that any given protection was a step too far, or that a particular regulation no longer makes sense given some sort of change in technology. But just handwaving it away? All of it?
When you say you've "always wondered", I have a hard time believing it. There are many books on this topic. Books, articles, podcasts, movies, blogs, everything. You can learn about the history of labor markets, the theory of it, the present-day reality. Your comment shows no sign of having done the slightest work to understand the topic.
Even if you are not inclined to study history, just try thinking about it as an engineer. Markets aren't magic. They are a specific technology for solving specific economic problems. There are conditions under which they work well, and conditions where they work poorly, sometimes so poorly as to fail. Like any other technology, they come in a variety of specific forms depending on need. They need to be properly installed and maintained if they are to serve the purpose they were created for.
Please go and learn something about this.
That said, tools like collective bargaining and unionization can bring it closer to the market ideal - workers acknowledging and taking advantage of their importance to the business to the same extent as the employer does.
In practice, many external if not anti-free-market factors have made that grouping harder to do.