Except that the airlines are often part of the problem.
If you didn't know anything about airlines, and just looked at historical on-time data for airports for the last decade or so, you'd think Newark had some major operational issue and just couldn't handle flights, because it was one of the most chronically-delayed airports in the US.
But the reason for the delays wasn't a problem with the airport's operations; it was a problem with United, which has a hub there. At the time all of the big traditional carriers in the US were expanding the use of higher-frequency flights on smaller jets operated by regional affiliates (as opposed to lower-frequency mainline service, which is more expensive for the mainline carrier). But United under Jeff Smisek's leadership went well beyond what anyone else was doing, and ended up pushing Newark to its absolute limit in terms of capacity. Result: traffic jams at the airport producing endless delays.
United should not be allowed to shrug and say "it's the airport's fault" in that case.
A similar thing happened in Dallas more recently; after the American Airlines/US Airways merger, AA switched to running DFW as a banked hub. The term comes from having "banks" of large numbers of flights arrive and depart almost simultaneously, rather than smaller clumps of flights arriving and departing throughout the day. In theory this is more efficient; it reduces the number of passengers with multi-hour waits between flights, requires less staff to handle flights during downtime between "banks", and helps maximize the use of the aircraft. But while US Airways had experience running banked operations, AA largely did not, and their staff at DFW simply weren't able to pull it off. Result: chaos at DFW, with flights landing and then having to wait 30-40 minutes for a gate to open up, and passengers missing their precisely-timed connections because they were still sitting on their inbound flight when their outbound flight pushed back.
While airlines love to complain and say it's the airport's fault, the number of genuinely capacity-constrained major airports in the US is surprisingly small. Poor scheduling and operations at the airlines are much more common causes of these problems.