Benzema isn't fit now. He is expected to return in less than a week. You can easily find his injury information here:

https://www.transfermarkt.com/karim-benzema/verletzungen/spieler/18922

It is going to be a month since his injury started. Otherwise there was no reason for France not to add him to their World Cup squad. Giroud is already very helpful for the team now of course. But if Benzema was here then Giroud would wait for his turn in the subs most probably. Benzema won a Ballon d'Or prize last season. He is still a very important player for not only Real Madrid but also France national team. Mbappe and Benzema has a really good understanding of each other in national matches.