Simply, Katy, because we live in the real world. Real world people use all sorts of terms that specialists in a particular field (like you and me) don't use. It's not me that has "decided to compartmentalise gulls", it's people out there in the real world. It's a perfectly natural and normal thing to do. People see the normal common, light-coloured seabirds and call them 'seagulls". Show the average non-birdwatcher a Pacific Gull or a Kelp Gull and it won't occur to them to call it a "seagull" - it's far too big, it's a completely different colour, it doesn't wheel around in vast flocks the moment you open a bag of bread rolls, and in any case it's not all that familiar.
Point one out to them and say "what's that?" and they'll say something like "I dunno. Is it an albatross?" And that's not as silly as it sounds - it does look a bit like an albatross, and it doesn't look much like an ordinary seagull.
There are all sorts of technically correct terms that pertain to parts of the human body. I can't remember what most of them are, not being a doctor (let alone spell them!) but when I do go to the doctor and say "Doc, I've got a lump under my arm" he doesn't say "Dont be silly, Mr Wilson, there is no such thing as a lump or an arm, it's a mild sub-cutaneous contusion located on the ventral surface of your upper forelimb". Instead he says "let me have a look at it".
(Usually, after a few moments, he adds "nothing to worry about, leave it alone and it will go away, that will be 48 dollars." - but that's an entirely different issue!)