Because the Author is Dead
Which is to say - interpreting literature doesn't really have much to do with intentions. A good author's intentions might be missed by pretty much everyone. A bad author's 'well meant intentions' might be hidden by inadvertent racism, casual sexism and so forth - or even just really obvious symbolism. Picking an exotic sounding name is all well and good, but when Space Captain Vaas Deferens has to launch a boarding party onto the Labia Rex...
Well, it doesn't matter what the author intended if, indeed, there's other indications visible in the prose.
O'course, it gets my goat up when folks say 'clearly they intend this' when it's not that obvious at all - but worded and supported well, some interpretations can be quite cracking. Nick Kyme's Salamanders series, for instance, about genetically enhanced supermen soldiers in walking tank armour... well, it parallels closely many of the Harry Potter styles and choices in a manner quite unintended (the author denies much of the parallels), but when pointed out... they're decidedly amusing.
(Right down to an outright 'yer a wizard, Harry' moment.)
Anyway, asserting it as fact is supremely irritating. Seeing a lot of the nuance and possibilities of interpretation, however, allows it to be very interesting to scrutinise too.