For one big reason, control! And all other reasons that come after that...

In theory, it's possible, in practice it's near to impossible!

It's like you are asking can someone conquer the US?! The answer is the same, can you imagine all countries to attack at the same time on all sides? But when we will see that kind of organization? Can we see so many people uniting, with enormous computing power, and trying to hijack the Bitcoin network?

In theory, things are possible because you can always find some ways that "maybe" can work, but practice and life are so different! In theory, any country can be conquered, any system can be hijacked/destroyed... I can become the president of the galaxy and League of Legends pro players... but I guess only in theory!