"hero" is pretty figurative & based on popularity! i can't say many specific people who are seen as heroes have had a very significant, if any, impact on my life.
there are american heroes like Lee or Washington. there are female heroes like susan b. anthony. but they wouldn't be recognized as worldwide heroes.
and then what if there's some amazing sonofabitch in malaysia or uzbekistan or somewhere that totally deserves the title, but i just didn't hear about it?
(also: i doubt that most of us are old enough for MLK to be a hero of OUR time. he died in '68. and, in any case, ghandi is probably "superior" just in sheer number of people affected by his efforts (but he died in the 40's - also assassinated by gunshot).)
not to mention--freely-available and prolific news (e.g., TV & internet) have made it difficult to focus.
there have been all sorts of bastions for causes in the past few decades, but their names are lost in the sea of information.
Let's think about it: Name 10 people who were exceptionally notable (and popularly notable) that were actively being heroic within the past 20 years. Or, hell, even 30 years. Actual names, not "that guy that did X".
nelson mandela famously called for an end to apartheid in south africa in the early 90's after decades in detainment.
mother theresa was pretty giving (died in ... '97, i think?). lived her entire life in direct service of the sick.
a lot of people thought princess diana was a hero (i sort of disagree but w/e). also died in the late 90's.
some people think ronald reagan was heroic for ending the cold war and calling for the destruction of the berlin wall. (but then some think he was the devil, lol). (u.s. president in the 80's.)
tenzin gyatso (the dalai lama) is a bastion for peace in tibet.
ever heard of shirin ebadi? a female iranian lawyer & judge, who worked her entire life against the subjugation of women. (still alive). (though i only know of her b/c i am an NPR dork.)
jack hanna & steve irwin (and even al gore) brought tons of attention to maintaining ecology & wildlife (which actually affects everyone on the planet indirectly)
probably wouldn't consider it, but what about Bill Gates & Warren Buffet? They have contributed UNTOLD wealth into charities ($30B+ USD). CARE Int'l, yearly scholarships, disaster relief, business startup grants, university funding, libraries, etc.
most our modern heroes just aren't very exciting to document.