//Why do people feel the need to come 'out'?//
The majority of people are straight, so people who aren't straight are typically assumed to be straight unless they otherwise say so.
This might seem trivial to someone who hasn't experienced it - but it can be a significant burden, because it means that everyone close to you doesn't understand that you fall in love with different people to them. And because there are still bigoted people around, it means that you're faced with the kind of anxiety the OP describes about whether your family/friends will still accept you if you tell them the truth - until you find out for sure.
I appreciate that to straight people this will all seem quite trivial but living through it is not, I can assure you.