Yes they are. There are various entities which give regular salaries to Core maintainers and devs to allow them to work on bitcoin full time. Wladimir van der Laan is employed by MIT under their Digital Currency Initiative. Pieter Wuille has been employed by Chaincode Labs and Blockstream. Blockstream also employ Andrew Chow. These companies all employ other developers too, as well as companies like Lightning Labs and Square.

Bitcoin Core development is only slow when compared to altcoins, most of which have no working product or anything of value to lose should they introduce some major bug or accidentally fork their chain.