Of course you can read these - if someone else has translated them for you.

There are nuances and things lost in translation that are often preserved in the original language. Someone who speaks/reads/understand the original language these documents and books are written has an advantage over someone who doesn't.

Furthermore, there is always the risk that the translations could be poor, or intentionally altered. For example, if someone wanted to trick me, they could claim that a document written in Hebrew or German says something that it does not. I would have have to find someone I trust to be honest who speaks these languages to help me know what these sources actually say.

This is why being able to speak the original languages primary sources are written in is an advantage over those who do not.

It's not that you cannot understand them. You can - as long as someone else does the work of translating it for you. That adds an extra layer of possible doubt when questioning the integrity or accuracy of the translation.