Because, of course, that's what it is. The genre is called 'fantasy' for a reason. Not every work of fiction is a more subliminal version of Animal Farm; some people write for the love of the art, of the creative process. That's why I used to write (before I discovered blogging), a hobby in which I drew much inspiration from childhood readings of Lewis and Tolkien. In any case both authors draw too extensively on pagan references, and are far too creative, for their work to be mere Christian allegory - a characterisation they strongly disclaimed. Lewis desccribed his work as 'suppositional' rather than allegorical:
Likewise Tolkien, who was perenially irritated by people's efforts to find direct representation in The Lord of the Rings ("To ask if the orcs are communists is to me as sensible as asking if communists are orcs"), preferred to speak of 'applicability' rather than allegory when discussing his creation. He and Lewis were story-tellers foremost, Tolkien having authored a distinguished paper on the subject. Both, however, believed that creative fiction should allow the reader to find points of connection with their own reality. The Lord of the Rings is a tapestry from which one can spin what one desires: greenies indulge themselves in luddism and talking trees, counterculturists take inspiration from the triumph of little people over abstract powers, atavists savour strains of Teutonic mythology. Or one can just appreciate it as the heroic epic that Tolkien wrote it to be. This is different from 'propaganda', which is designed to influence the consumer's opinions.
"In conclusion, remember to kill anything that isn't human and worships the Emperor. Trust only bioengineered supermen like yourselves. Hatred is righteousness, difference is heresy and individual life has no value. Right then, let's off on Crusade!"