For at least two major reasons that come to mind immediately (no doubt there are more):
First, breaching a login info database for a web service isn't the same thing as actually logging into an account or accounts and using the service. For example, if I were to somehow breach Amazon's user authentication database (Which I'd never try to do, and which I'd never ever, ever possibly succeed in doing even if I tried.) that would be a very different thing from actually getting in and ordering things as someone else to my heart's content, or buying & watching movies online, or whatever. Similarly, as someone else mentioned you would want someone's bank site password so you could then log-in and perform some transaction that benefitted you, or copy down some secret financial payment info you could use to commit a financial fraud some other way, or do something along those lines. A password hash--or even a password itself--is just a pointless string of bits if it doesn't help you get access to something somewhere. And if you want to abuse someone's account to gain access to a given service, you need to un-hash their password hash back to the original to do so. And do so before the service realizes their user auth database has been breached and resets all the exposed passwords.
Second, human beings, in the aggregate, are pretty poor at using passwords properly. A lot of people--a small minority, relatively speaking, but still a lot of people in absolute terms--will choose incredibly weak/guessable passwords, as I'm sure you know. But an even larger minority will reuse passwords between different services & sites. And even reuse passwords between important services and sites vs. much less important services and sites. So, if a hacker breaches the user auth database for megastupidsportsaroundearth.com and gets (at the extreme) email addresses and cleartext passwords for a million people, it's a good bet thousands or even tens of thousands of those log-ins will also get you in the door at various banking sites, major online retailers, email services, etc, etc. around the net too. So even if megastupidsportsaroundearth.com invalidates all the passwords that were taken immediately after the breach occurs those email address and password combos will remain active at all the other sites where the user was using them. (Unless, maybe, your bank has a cybersecurity operation that is actively trawling the darknet for collections of stolen user credentials for its site and canceling them before bad guys can use them. But that's unlikely to save you even if your bank happens to be one of the minority who is doing such things so far.)
Password reuse with important accounts: just say no. :)