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/18922It 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 have a really good understanding of each other in national matches.
It is very unfortunate for the France team not to have Benzema in action. On the other hand, they do have enough replacements for him in the squad to cover his absence. Olivier Giroud has been playing really well in the tournament and there is nothing to talk about Kylian Mbappe as he is the top scorer of the tournament so far scoring five goals already.
The addition of Karim Benzema would've been great for the French people as he has been in top form with Real Madrid so far and also with the France international team. I hope that Giroud and Kylian Mbappe fill the shoes of Karim Benzema well enough because this France team looks like it has the potential to defend the trophy at this World Cup.
Regards
Duke