There's another compelling reason for NOT merging airlines from different countries. At the moment flights between pairs of countries one of which is outside the EU are mostly still governed by bilateral treaties between the countries concerned, allowing only airlines domiciled in those countries to carry traffic. If you decide to merge, you automatically lose a significant part of your network. The EU is busy negotiating treaties that allow airlines domiciled in any EU country to fly from any other, and of course this already happens for flights wholly within the EU and for destinations as varied as the USA and Morocco, but it will take years for the whole world to be covered by these agreements.