it's not really that we don't have those types of fighters now a days in my opinion. it's more of the fact that the fans and media are so quick to crown the next guy a king without allowing him to prove it himself, and when that guy fails it just further waters down the title of "greatest" or "GOAT" or "p4p king".
all of those guys, GSP/Fedor/Anderson went on huge winning streaks, we have guys like that currently, Khabib/Holloway/Mighty Mouse but media and fans don't really care because the environment has changed. people want things RIGHT NOW they don't want to wait 10 years to see if Khabib is still undefeated, they don't want fighters to take their time and accrue the needed experience one would need to accomplish greatness, they want everything fast tracked so they can say "i told you so" or whatever.
everything in the world moves 1000x faster than it did when GSP/Fedor/Anderson were at their peaks, more social media, more exposure of the sport, more people connected to the internet, etc.,
but it's like that famous quote goes: "You can't rush greatness"