Or it could be that owners of internet infrastructure are never sued because they have legal departments that can spend a lot of resources fighting back, keeping the copyright holders laywers busy (and draining resources) for a long time. On the other hand, end-users don't have legal departments, and probably don't have the resources to pay for a single lawyer for very long, and are much more likely to settle to make things go away, so it makes much more business sense to sue them.
The DMCA has explicit exceptions very clearly spelled out for service providers that do not store content and/or do not choose which content gets cached...
Which absolutely sucks. But there it is.