because the phpBB size limiter for off-site avatars doesn't work. you can set it to 1x1 and load a 42379x234789 avatar with no warnings at all. this would destroy the forum. a gallery is the only way to go.well, there are some java-resizer for custom avatars, but they make the code heavier.
in fact, you can do that, but the problem is, that the browser download the whole pic and resize it after. that mean if everyone (like some stupid forum i know) use a wallpaper size pic, that slow a lot the navigation.