Why does it matter if hashes are the same?

That only tells you the passwords are the same.

The point here is that once you brute force the plaintext password, the same password might be used elsewhere.

Password salting has nothing to do with password reuse.

Imagine two people have accounts on each of two websites:

             eBay           YouTube
   
   Alice     sunlight       bobrules
   
   Bob       bobrules       bobrules

A password reuse attack dumps the YouTube database, cracks Bob's password, and then accesses Bob's eBay account. The fix for this is that Bob should use different passwords on his different accounts. Hashing helps by making step 2 ("crack Bob's password") more difficult. Salting does not affect this attack in any way. Note that the attacker didn't bother to dump the eBay database.

The attack that salting protects against dumps the YouTube database, cracks Bob's password, and then accesses Alice's YouTube account.

You don't brute force it, you find the password for accounts with the same e-mail in leaks from other sites and try only those.

You can still run those "top billion popular password" lists against properly salted/hashed passwords.