As said before, AI's can choose their appearances. The Rampant Cortana that we see in Halo 5 chose her form because she wanted to show that she was powerful and command respect, in keeping with her plan to subject the galaxy to Imperial Peace which amounted to a gun pointed at the head of the universe and saying 'obey'.What, thats dumb. Everyone just chooses how they want to look but it never reveals why. Why does Roland need to look WW2 pilot, I think its just their programming. No need to look like that.
It really isn't their programming. It is stated in canon that AIs choose how they want to look based on a number of factors. Part of it is, yes, the role they are designed to do, but also something they admire.
One AI, for example, looked like a Picasso painting, or an abstract star field that blinked when the AI spoke. They choose how they look. Most choose to look human because it makes interacting with humans easier and more comfortable for them. Others choose to look like, in the example of a ONI AI, like a grim reaper.
You may think it's dumb, but it's actually pretty neat, in my opinion. It shows a degree of freedom and originality and allows the AI to have some depth of character. If you can't understand it, that's fine, but it isn't dumb, and it is very much canon.