For pretty much the same reason that most of Spanish fans hate Real Madrid, most of german fans hate Bayern, most of Italian fans hate Juve.
These are the biggest clubs in their countries and have the highest fanbase. At the same time, there are more people than hate those clubs that the others.
Fact is, a lot of English fans love to hate United and think that United is their No.1 rival, just because United is so big and popular. Our rivals have always been Liverpool, City and Leeds (in that order), but all three of them consider us as their biggest rival. And clubs like West Ham consider us as their biggest rival, despite that we don't care about their existence.
It is the same thing here in Italy. Juventus has as biggest rivals Torino and Inter. On the other side, the likes of Roma, Fiorentina, Napoli and to some degree Milan, hate Juventus with a passion of a thousand suns despite that Juve don't give a shit about them (most of the time). At the entrance of Firenze in the highway is written on extremely large letters 'Juve AIDS' and Fiorentina fans hate Juventus so much but on the other side, Juve fans are mostly indifferent to Fiorentina. Similarly, to how we are indifferent to West Ham, or Real Madrid is indifferent to some midtable Spanish clubs that think that Real is their biggest rival.