There's reasons beyond just legality. If you are in a company which has a culture of firing people at any moment for no (obvious)reason, you're going to see high turnover and low morale as nobody wants to work with an axe over their heads, particularly in an industry where it's hard to replace people.
Sure, but that doesn't mean that any one person is protected from, say, a capricious and arbitrary manager — and it's absolutely legal.
In any case, saying "a company cannot fire you just like that" is patently false in jurisdictions with at-will employment, which is what I was pointing out in the OP's comment.
“As long as the reason is not illegal (e.g. firing because of the employee's race, religion or sexuality.)”
While the odds of someone sueing a company for that are very high, the costs are also extremely high. Companies do extensive performance evaluations, performance improvement plans etc, to minimize the potential for wrongful dismissal lawsuits.