Because we can draw what we want. Since this is just fanart made for our own enjoyment, we draw what we pleases us, aesthetically or otherwise. I personally am not a fan of Alice's default clothing, so I drew a blouse, blazer, and boots. There is one more personal reason for why I do it though...
Shortly before I discovered Reborn, I created a character design that looks eerily similar to Alice. When started playing the game, I substituted Alice in for my OC, so when I draw the blonde protag, it feels wrong to draw Alice and not her lolol
In short: Because artistic freedom