"They" (meaning the spammers) send out millions of emails. At first, the domains et al work.. By the time you receive one, you'll be number 12377654, the first few might already reported it as spam. Responsible people took down the domains before you had a chance to fall for it..

This leads to my second question - are there any drawbacks to using reverse dns?
Numerous.. Split in and egress mailservers for one. The outgoing mailserver might not be the one receiving email. Hence there's no MX record for it. Servers setup like that would get flagged as spammers..