why is market clearing price assumed in cournot model with homogenous products
[...] 
   One way to interpret this result is to see our two-stage game as a mechanism to generate Cournot-like outcomes that dispenses with the mythical auctioneer. 


A justification is given in Kreps-Scheinkmann (1983). They argue that this is the outcome if price competition follows production. For details see the linked article.