Why is cooking rice so complicated? I'm slowly teaching myself to cook and I made some rice today. While the result was perfectly acceptable, the process didn't go smoothly and I had to add some water in the end and cook it some more to get there. 

I did follow the process on the package of rice, though I also read a bit about rice cooking in general before that. So I added the rice with a defined amount of water in a pot, brought it to boil and then simmered it with a half-closed lid for around 10 minutes.

What I'm wondering now is why rice is cooked in this way, with a defined amount of water added and then until the water is absorbed or evaporated? The process seems to me to be rather fragile as the result depends a lot on getting the rice/water ratio right. 

Compared to cooking pasta for example, the process seems to be more finicky and depending on rather exact measurements. So why exactly isn't rice just cooked like pasta, where you bring excess water to a boil and put the rice in for a defined amount of time, with the option of tasting when it is done?
Once you get the hang of it, rice is as easy as pasta. One thing you say in the question that may be central to the difficulty you are having is that your lid is "half-closed". For the majority of rice cooking methods, not only should you keep the lid tightly closed, but you shouldn't even open it to check the rice until it has cooked close to long enough that it might be done. Look at the accepted answer here: Rice gets burnt and watery. That answer explains it more thoroughly.