Why isn’t QuantExt a part of QuantLib, the two projects seem very similar.
We have chosen to use QuantExt for two practical reasons. The first one is flexibility: we want to be able to add QuantLib extensions when required for ORE without being tied to the QuantLib release cycle and process of PR reviews. The second reason is that the extensions may be a bit specific around market simulation models so that it is not clear whether QuantLib is the right place. In any case, we are open to establish a process of migrating QuantExt content over time into QuantLib where it makes sense. We cannot even avoid that volunteers will copy code across (it is open source!). We will discuss that on the QuantLib mailing list.