I don’t think they are. They are!
Paystack is essentially a “reseller” of FlutterWave’s excellent payment tech. The main product they “resell” is the FlutterWave Pay Button. FlutterWave’s API allows you to do some wonderful things, that’s why some of the best services use them - e.g. Uber, KongaPay, PayWithCapture, etc.
Think of it this way. FlutterWave is manufacturer of the product, Paystack is the retailer, and there are other retailers, you can become a retailer too if you are interested in fintech.