Because it doesn't use oil. I do the same with my daily cars, they don't use oil, so I don't check. Once you get a idea the engine doesn't use any noticeable amount of oil you can stop checking!
Im your case you car burns a fair bit if oil, so you need to keep an eye on it. To me that's a mental amount of oil, but really it's not like you are going to rebuild the engine just because of that