In the UK you can be fired within the first two years without reason (unless it's connected to a protected characteristic). After two years you can still be fired - the company just has to follow a simple procedure of giving you some warnings before firing you.
Employer tribunals are not free and are intimidating to use, so many people who are wrongfully dismissed don't seek justice.
Having had to fire someone who had been at a company for more than two years, I can say it was anything but a simple procedure: it involved a performance improvement plan which lasted interminably (and actually sapped a really surprisingly large amount of my own time...) and it became an awful lot more complicated when it turned out the guy was on anti-depressants. In the end, HR suggested I take the guy to the pub for lunch and they told me a series of things I could say to encourage him to quit but which couldn't possibly be construed as constructive dismissal; thankfully over lunch he told me he'd got another job offer, and I walked him out of the office the following day with a great sense of relief.
Employer tribunals are not free and are intimidating to use, so many people who are wrongfully dismissed don't seek justice.