There are two reasons for this:
1) The letters need to be written not scraped. it says in the Torah Uchesavtom "you shall write" not :you shall scrape: since the Reish and Vov lost their form in creating the Ches we cannot just separate them because we would be recreating these letters through scraping as opposed to writhing.
2) We cannot erase these letters completely to rewrite them because a Mezuzah needs to be written in order. It says “Vehoyu” and the Medresh explains “Behaviosom Yihiyu” which means that the order that the letters are in the Torah is the order that they have to be written in, so if for example the Sofer wrote the word of Shemah and then he realized that the shin was Possul he cannot just correct it he must first erase the Ayin and the Mem then correct the Shin and start again. (If Hashems name is written after the mistake it cannot be corrected, because in order to correct you would need to erase all written after the mistake and Hashems name cannot be erased) in this particular case the letters are technically not Reish and Vov but it is a Ches. If we were to separate them or even erase and rewrite them we would be creating a Reish and a Vov "out of order"