Think about what that would take in terms of man hours to accomplish moving from EL6 to 7. And moving from 5 to 6 was not much better. This is just too expensive to repeat every three years.
So why do it? There is absolutely nothing wrong with sticking with EL6 for a long time, certainly for the lifetime of the hardware - EL5 has only just gone EoL, and if you pay RH you can still have it on support. Just because EL7 exists, it doesn't mean that you have to upgrade to it. I've only just started to seriously roll out CentOS7, and then mostly only on new machines.
P.