Because there are still stories to be told in this universe. Telling those stories in the form of an original title would approach the threshold of plagiarism, because they would require elements fairly specific to the Alien universe.
And it can be argued that walking back the last few movies is actually embracing those restrictions and ditching "mindless wanking" and "floaty bits without meaning." The first two films gave us a universe that was fairly coherent. The next few gave us a universe where space monks contemplate God in a near-abandoned prison, xenomorph eggs magically spawn out of nowhere, and clones have the memories of their originals.