There are lots of image sizes that I use on the website that aren't supported on the API I don't want to be responsible for supporting more than I realistically can. Every new image size introduces another level of risk when I add or modify the sizes I use. It's also another size that has to get created and cached, and there's no guarantee where I'm using the size is where you will (ie. I might only use it for a particular media type) which means the cache hit will be low thus we would be delivering slow images.