This stems from what Tridus eluded to. The developers have essentially backed themselves into a corner since the only threatening mechanics is things which will kill you outright or come incredibly close. Even God Kefka's aoe damage isn't the frightening aspect. It's the fact he reduces you to 1HP then spams raid wide aoes. Likewise, both he and Exdeath used gimmicks that require everyone to be at 100%, thereby forcing more healing. If healers couldn't bring you from 1 to 70,000 in seconds, you could have less OHKO mechanics as damage would be threatening again.
