So, I've looked online and all I can find are the positives of joining a union and some satirical articles on why you shouldn't join.
Are there any negatives, and if not, why doesn't everyone join?
I've been in several unions.
I am in the USA, so these points may or may not apply to a greater or lesser extent in the UK.
Here's what I did not like.
The union cares about the union, not you.
Many make the mistake of thinking that the union protects it's members, it does not. The union protects itself. Case in point. One union shop I worked at stood their ground on reduction of benefits and salary. The fact that many of us would have gladly taken cuts and stayed employed was irrelevant. End result, layoffs.
You all get paid the same.
Do a good job, and you get paid the same as the guy who is goofing off, maybe even less. A rise in pay in a union shop is by seniority, not by merit. While there is collective bargaining, there is also collective results.
Achievement is discouraged, ambition is not rewarded
When I worked in a union, I was ambitious and worked hard and was rewarded with hearing "You're making us look bad". In a union shop, if you perform too far above your coworkers, you WILL be singled out, and not in a good way
Collective bargaining can bite you
If the union calls a strike, you're out of work for the duration and you NEVER make that money back. Decades ago, unions used to keep "Strike funds" to pay the lost wages to the striking employees. That is no longer the case.
Another layer of bureaucracy
While every environment has "office politics", a union adds another layer to the mix. Not only do you need to navigate the politics of the company, but the politics of the union as well. Annoy the shop steward or any key union members, and you're going to be miserable, and the union will make sure of it.
Disobey the union, and you're going to pay
I've seen this first hand. get the label of "Rat" or worse, and the rest of your time is going to be miserable. Expect vandalism, being frozen out, posters on your locker. Your locks glued shut. Not fun.
You don't join a union for free and union dues run into the thousands per year, depending on the shop. You also have no say in where these dues go. If the union wants to throw your money into causes you disagree with, too bad for you.
Off the top of my head:
There are various other arguments you'll read for and against joining a union and a lot of anecdote but these are two concrete things to consider.
Here are some reasons, on top on my mind, on why you might not want to join one. This list is not exhaustive at all.
Do you have the time to join a union and actively participate ? A lot of people who already have their jobs (including travel time) and families do not have enough time to invest themselves in a collective action.
Some unions will ask you for a subscription / donation. You might not be willing to pay this fee.
You will be flagged as "a unionist". This means that you will have to go against the management in a lot of cases, and some people will have a bad opinion of view (I am not discussing whether this is right or not, but a lot of people do not like unionists, because they see them as lazy / anti-productive).
As in any organization, when you joined, you will not be assigned the most interesting tasks, which might not encourage you unless you do have a strong interest in unions.