I think the answer may be simple:
- God is maximizing the population of heaven. Why stop the Earth ecosystem prematurely if it can still bear more fruit?
- God is giving people alive now the chance to repent.
Hence, Jesus will return when the last saint has been born and when the last person who will repent has done so.
Also, we only know what is happening here on Earth. God may have involvements with other creatures in other solar systems; these points may apply to them as well.
Also, we don’t know the details of what relationship God has with animals, insects, other life on Earth. He may be doing something for their sake as well that we don’t know about. It might be Church teaching that the souls of these beings perish at their death, but even so, we don’t know the implication of their lives for the universe that will be reborn/perfected according to St. Paul, and God could always bring them back, even if they ceased to exist from our perspective. After all, if God sees all moments of physical time immediately, then “a few blank frames” (the period of time where something doesn’t exist) “doesn’t matter” to Him, because He can interact directly with them before and after those ‘blank frames’, establishing continuity through Himself.
I may have complicated my simple answer, but I think the two points I enumerated above answer this question: It makes sense for Jesus to “wait” until the population of heaven is maximized. Moreover, didn’t He say in the Gospel that He was “going to prepare a place for [us]”? So if He’s doing something in a spiritual dimension, then perhaps we should get busy doing things in this physical dimension, preparing for our arrival there (i.e. bettering ourselves, working to end injustice, evangelizing our neighbors).