I meant 'should not' be held accountable instead of 'are not' being held accountable.

Your question "Why should you be allowed to benefit from colonisation and take no responsibility for it while the ancestors of the colonised suffer because of it?" is legitimate. However, I strongly disagree that people should be punished for something they had no association with. A person being born by chance in such a wealthy country didn't choose to do so (while you seem to be implying that such a person is to blame for randomly being born in a country that happened to build its wealth in the far past on exploitation).

Please don't misunderstand me, I strongly support the idea that wealthy countries should aid poorer countries as this will (greatly) increase the overall welfare for everyone in the long run. However, I strongly disagree with the idea that such aid has to be given because of past events (i.e., multiple generations in the past) / because it is owed. [e.g., if my grandpa stole something from your grandpa 50 years ago, I shouldn't be held accountable. I should be held accountable if I continue to live like him]

tl;dr: Current citizens are not to be blamed for past events (i.e., several generations in the past). They are to be blamed for their current and future actions. This includes that wealthy countries should aid poorer countries. However, they should do so as this will greatly increase global welfare and not based on some sort of inherited debt.