It's not a fascination, it's that these famines are used as cheap "gotcha" arguments against socialism without any regard to the context surrounding them.

Mistakes were absolutely made during both examples and people died because of those mistakes. At the same time, both serve as examples of how planned economies DO work once taken in a larger context. Yes, both societies had a famine and it was terrible. That's not the end of the story, though. If we are talking about these famines in regards to which is superior, socialism or capitalism, there are other questions to consider.

How many famines did each country have after these examples? How frequently had similar famines been occurring prior to the reorganization into a socialist system? Was this solely the result of mismanagement by the socialist governments or did the conditions created by the systems in place before the revolutions contribute to the famines? Did similar famines occur elsewhere in countries organized around capitalist systems near the same time? Why is there so much focus on the (unintentional) famines that occurred early in socialist countries when far less focus is put on (intentional) famines that occurred in capitalist/colonized countries? Are we giving socialism fair consideration in this discourse?

This rabbit hole goes plenty deeper. The frustration is not originating from the attack on societies that people are fascinated with, but that these are used as bad faith attacks that are taken out of context. They put socialism on a false pedestal and then kick the legs out from under them.

As for Cuba, I can't think of a better example of socialism being a viable system after all their people have had to suffer through. There hasn't been a single day since long before the Cuban revolution that the US has given the people of Cuba a fair chance to live a prosperous and peaceful life. Despite the relentless pressure to fail, they persevere and in many ways thrive.