H-bridge with BJTs: why do IC and discrete solutions differ (or Sziklai vs Darlington)
An NPN darlington can be made with the collectors of the two NPNs connected together. In transistor technology, this is the substrate (basically the starting point for the transistor construction). Sharing the collectors allows relatively cheap construction. Basically, the transistor is constructed from a 'holder' layer that is the collector; separate bases are constructed simultaneously in this, and separate emitters are constructed simultaneously in each of these; 

A composite darlington (Szilaki) requires more complex construction with the PNP's base needing to be distinct from the NPN's collector (or emitter) structures. 

In an IC, there are already many different semiconductor layers (for constructing these PNPs and NPNs), so the structures are already available.