You mean the national tests?

Sure, we have those, they take place in your normal school at the same time all over the country.

Ok, say a child is homeschooled for five years, and then fails a national test, that means that they have effectively lost a lot of their best time to learn, and they need to retake the subjects to catch up.

Proper schooling allows for students with special needs to get extra help.

I was one of those students, I was on the spectrum of having light autism, possibly some ADHD/ADD and similar stuff, I got an assistant teacher in school for several years, and even went to a smaller class specifically tailored towards students with my kind of mental issues, as I grew up I became more independent and learned to live with my difficulties, these days I don't consider myself to have any real problems from my earlier diagnosis, and can even use them to my advantage.

I got the help I needed, when I needed it, while attending mostly normal classes and getting socialize with fellow students.

The socializing part was hugely important for my development, without that I would not have been able to go as far as I have.

When I look at homeschooling, I see it as parents denying their children a proper education, often to force their own warped worldview onto their children, this obviously comes from someone who experienced a well functioning school, even if I was bullied at times.

TL;DR: Testing only shows the result of time spent in education, it can't catch students as they are about to fall through the cracks as a proper teacher can during lessons, this means that a homeschooled student that fails the test has lost a lot of opportunities to get extra help during their schooling.