If say a goblin tribe is massacring farmers in the eastern provinces, then a powerful wizard could simply teleport the party there. And if he can do that, maybe help with scrying as well? And maybe scry first, teleport there second, fireball the lot of them third?
All of these cost time - time he doesn't necessarily have.
"I've heard some reports of a goblin tribe in the province of Asfalah attacking local farms. Look into it, discover the causes, and put a stop to it if possible. If it's just a minor population boom, you should be able to handle it. If it's beyond your ability to cope with, report back to me.
Here are the components for two
Linked Portal rituals, as well as the sigils for the local wizards' guild - the guildmaster owes me some significant favors, so don't take lip from him. Good luck! Now excuse me, I've heard some disturbing reports from the Underdark that I need to look into..."
After all, lives are at stake.
Lives are
always at stake. The question is where his powers are the most useful. Out there in a frontier situation to deal with a situation lower-level adventurers could deal with? Or investigating the schemes of some of his truly powerful enemies which might threaten the entire kingdom?
He can't be everywhere at once.
And doesn't he have asisstants who can handle red tape for a day or two?
He has assistants. So do corporate executives and heads of government. But assistants can't really make the high-level
decisions for them - which resources to spend, what to protect, where to make a difference.
Once the PCs are past the beginner levels, and the scope of their adventures is widening, then it gets harder and harder to justify NPC inaction.
Who says they are being inactive? They are providing the PCs with resources and information. It's just that the crisis the PCs are dealing with right now is unlikely to be the only crisis at hand. And which crisis is the more serious one is often only discovered when it is too late.
That's why I prefer not to show players that ruler X is a high-level whatever. Less "but as a paladin he cannot let those people die! And he has a wizard friend who could help too!" arguments - which are often perfectly in-character. The NPCs can be tied up in red tape too, but I'd rather not enter discussions whether or not threat X should be enough for the hero NPC to change his daily schedule or not.
As it happens, these complaints echoes the complaints of military grunts throughout the ages. "General X spends all his time and resources on unit Y! Why doesn't he spend it on our unit? Can't he see that we are much more important?"
But of course, none of the people on the frontline have enough knowledge to see the big picture. The same is true for PCs who haven't reached the level of the NPC yet.