In my own words: A labour union is a group of workers who have banded together for better pay, benefits, and conditions at their workplace.
If you like those things, you should organize a union with your coworkers. If you already have those things, you should organize a union to keep them.
Of course, if only it were that easy! To keep it brief, here are some reasons why people don't "just organize":
Most people support unions. But other people, especially younger workers, don't even know what unions are. (I didn't know what they were until my mid-20s!) But the general idea is that if there are less people in unions, then less people know who union workers are, let alone their unions. More importantly, less people know what unions do for workers.
Another issue is the time and effort it takes. Employers are good at squeezing hours and energy out of workers. To maximize their profits, they minimize employee wages, and limit downtime: breaks, rest, leisure, even meals. Employers hate unions because it means they'll have less control over those things, and therefore less control over their own profits.
Employers usually earn a lot more than their workers. They use that money, time, and control over workers to fight union organizing. Union busting. Your boss might intimidate you, or create divisions between you and your coworkers.
I've seen a coworker mysteriously get a raise or benefits, while others got nothing. Or an entire staff getting punished for something one worker did. Even reprimanded for speaking another language during a break. The list goes on.
It's pretty damn hard to organize. Workplaces that need a union the most are often ones with the lowest paid workers. They don't have a lot of money saved, so they really can't afford to lose their jobs. Wage work also means lower job security in general. The system is rigged against workers. Even if workers win a union vote, employers can stall the first contract and make their workers' lives awful in the meantime.
But unions have to start somewhere. The 8-hour workday was once 10, and even longer before that. It took time and effort from workers to fight for better. Things can change, and they should.
The workplace is where most of us are. Us being people who have to work for a living. We give the best hours of the best days of the best years of our lives to employers, as economist Richard Wolff has said. It's worth improving the workplace for yourself and your coworkers.