> assuming focus is a limited resource

Focus (on a society-wide level) is not so very limited. One of the strengths of our whole society being fragmented into a bunch of distinct organisations, is that they can all go off and solve different problems at the same time.

> Shouldn’t problems be addressed in order of importance?

Most of the really big problems facing society can only be addressed through multi-solving (i.e. something like climate change isn't going to just hang around waiting for us to solve one sub-problem at a time). Not to mention, the solution space is often extremely interconnected - for example, finding alternatives so that society can eat less meat helps address both climate change and the horrors of factory farming at the same time.