This is a very basic fact of life that everyone should learn in school:
If you are forced to accept the bare minimum that is put on the table, your capacity to say no quickly crashes down, to the point that you may be vulnerable to accept a very unfavorable deal.
A scenario where the vast majority of us might find this reality at some point through our lives is the labor market. Whether you are applying for a job, or requesting a raise or a promotion, you are only going to have leverage to get the company to offer you a better deal if you have better opportunities on the table. In socioeconomic contexts where wages are depressed, this is usually not the case. This means that, for a lot of people, accepting a very bad offer means the difference between living a miserable life with a roof over your head and becoming homeless, so they do virtually have no choice but to accept, which only becomes more apparent if they have family members who depend on them.
It is interesting to note that this may be taught in detail to students of business, economics and law, although it is important information for everyone who participates in the economy: https://www.pon.harvard.edu/tag/batna/
In this context, a labor union that decides to initiate a strike isn't just provoking trouble for the sake of it - it is leveling the playing field by creating a situation where not only the livelihoods of the workers are dependent on the negotiation, but the profits of the company and even its capacity to survive are as well, whereas the latter usually wouldn't be.
Note that this applies to many other aspects of life as well. People often stay in abusive relationships because they do not have the means (or think they do not have the means) to leave them. It is difficult to leave the household you share with an abusive partner if you do not have the economic means to move out, and some people may stay in disfunctional friend groups because they think they aren't capable of making new friends, but need some social contact nonetheless.
Different configurations of society may protect people from these pitfalls or incentivize falling into them. The idea that people should find the means to leave their parents' household as soon as they turn 18 deprives them of an economic mattress that would otherwise allow them to be more aggressive when they negotiate for their salary, or even open up the possibility to dedicate time into trying to create their own business or projects. Different forms of social security, such as unemployment benefits, minimum guaranteed income or basic universal rent make working people far less dependent on the possibility of being laid off, which would motivate them to confront management about negative working conditions.
How about, the UBI program by law becomes a partial shareholder of all publicly traded companies in the country.
Unfortunately, that would be easily gamed. You just have the publicly traded company based outside the country. It licenses a ltd company for the rights to various stuff. This is a common, legitimate configuration for many businesses. This would bypass such a law quite trivially.
Setting aside specifics, the idea would be to gate access to the market contingent on the UBI program being entitled to a portion of whatever investors are entitled to. Do you really think there is no possible way to use whatever leverage the nation has over what businesses need access to, to pull that off?
Considering how hard it is for governments to get taxes out of multinationals right now, it's hard. Big business has a huge vested interest in finding tax loopholes and exploiting them ruthlessly. They also have a strong interest in disrupting any changes that will hit their profit margins. This includes fighting against law changes, both by lobbying, and in public opinion.
That's fair, but I think ultimately that's where the wealth is, and companies do need legal permission to operate. There are a lot of issues trying to do UBI with only income taxes.