I'm not sure why you think that. At least in Germany, if an employee can't do the work they are obligated to do by their employment contract, they are in violation. After they have been notified of the violation (or if it is obvious, e.g. because they didn't even go to work), it is their duty to correct that. If the employee is unable to satisfy the requirements, they can be terminated with an appropriate notice period.

(Termination protection does not apply if the behavior of the employee is the reason: http://www.gesetze-im-internet.de/kschg/__1.html)

The process may not be fast, but for a useless employee, it would be difficult to claim that the termination was unjustified, so the outcome is essentially guaranteed. I would call that easy (as opposed to simple).

Yes ... and this is a major drain on EU companies that make them less competitive than companies with "at will" employment. They can't fire low-performers easily, and they can't afford to take a gamble on people since firing them is so difficult if they don't work out.

That's mostly propaganda and the fear of "socialist" Europe its actually quite easy to remove people for poor performance assuming it is really poor performance or trying to avoid redundancy.

If its so easy to fire in the USA why is hr so paranoid about it

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.

It's not specifically an EU issue. The UK is a lot more like the US in that respect, as is Ireland.