You say it like that, but the reasoning is entirely up to the DM and they can easily use whatever bits of lore they have from their own worldbuilding to make reasonable problems spring up in a reasonable timeframe.
Between low-level adventures, there can be a week or two of downtime which isn't that long, but the crises aren't as world-threatening. At high-level adventures, decades could pass as downtime and the opinions of several powerful, ancient beings could change. The once peaceful dragon is seeing how the dwarves have done nothing but ruin the mountains he once created. The archdevil's plans have finally begun formulating. The Gods have one last trial to put them through before they are ready to challenge the evil god of murder or something.
High-level problems will find the players since they've become so strong and recognizable that powerful, intelligent creatures know they'll need to destroy them to carry out whatever they need to do. They were nobodies at level 1 and they weren't even on the villain's radar, but level 17+ adventures are threats that must be countered directly using intel-gathering, countermagics, and deadly force.