There is no obligation to leave your wealth to the poorest person...

No, but we do require wealthy people to pay a significant proportion of their estate in inheritance tax, which at least in theory is used for the good of the community as a whole (I realise the execution is deficient.) I also think one important difference between organ donation and financial legacies is that the latter require negligible public resources (which are more than offset by the tax), whereas organ donation requires considerable resources from a health service that is already stretched. It seems important to me that as far as possible, the allocation of communal resources should be prioritised by need and likely outcome.