There are rules about how far from an airport a plane can be, depending on number of engines (redundancy) and how it is rated. If you go to gcmap.com I believe

There are rules about how far from an airport a plane can be, depending on number of engines (redundancy) and how it is rated. If you go to gcmap.com I believe it gives ranges for ETOPS which is a rule for big twin engine planes. AFAIK this is one reason why there are no flights over the south pole (the other being it's not on the way anywhere). Occasionally I feel like I have seen it mentioned that a plane had to deviate from the shortest route because of being too far from an airport. Also, I recall reading that for Easter Island, only one plane can be inbound during a given window, because it is remote enough that there is nowhere to divert to. But you'd have to check to make sure I'm remembering correctly