Hi Francois,
this way we are independent of the QuantLib release cycle and can introduce missing pieces whenever we need them. Another reason is that some classes in QuantExt may seem too specialised for QuantLib, like the InterpolatedDiscountCurve variant which was optimised for the XVA simulation run.
But it’s not impossible that the two libraries might be merged in the future, let’s see.
Kind Regards
Peter