Perché questo caso è un "si" impersonale, che viene usato per indicare che un'azione è compiuta in senso generale. Ad esempio, "In Italia si cena alle otto" ha lo stesso valore di "In italia le persone (in generale) cenano alle otto." Spero sia comprensibile come spiegazione!

Thank you for reading my comment. :-) Matteo's comment is very enlightening, indeed: coupled with his answer, it gives you almost all you need. I will just add that the agreement has to be in grammatical gender, too; an example of agreement in both number and grammatical gender is "Dopo un parto difficile si è spossate", where, obviously, 'si' refers to women and the agreement is in the feminine. In your example the verb belongs to the class of "verbi copulativi" and "lettori forti" is what we call "complemento predicativo del soggetto", so that agreement with the direct object is impossible, simply because the sentence doesn't comprise one.

Thanks for the explanation and sorry that I closed the question too early. I thought Matteo's answer was sufficient.

In your sentence the subject is 'si': this is the "si impersonale" operator, which always takes a singular verb. If you had a direct object expressed, things would be different. You may want to wait longer before closing questions.