It really makes sense to use one of these clones for creating new products IF license permits it and if they are implementing a lot of required functionality. For example making a website for campsites would probably be 90% finished if an Airbnb's clone's code were used.

Just as we wouldn't think to implement Quicksort 10 or 20 years ago, because we have great, optimized reusable components, this is the next logical step: Focussing on building the 10% that differentiate the product instead of reinventing the wheel.