He is considered as such by most, you know you are part of the little group of haters who don't
You don't compare different era's, former champions aren't greater than Alberto and vis versa.
They were able to win both classics and GTs? Well it is almost impossible nowadays, the overall level is
much higher and riders are so specialized for a specific type of race that it has become unthinkable to win on every terrain .
So, he is a legend for what he already won in the current era (still going strong for a few years), for his great climbing-attacking style, for his popularity, for the drama and so on.
Those criterions also applied to champions like Hinault, Anquetil ect. You'll end up noticing there's nothing different actually.