The simplest and likeliest explanation is that she was indisposed until recently. And, that Fury only just recently learned that she was back in action such that he could page her.
This explanation is neither simple nor likely.
She was indisposed...for 25 years. Not on earth, not in Kree space, just elsewhere. Nobody knows. Freed up just in time to watch half the galaxy die because of Thanos. Who has been rampaging in space mass murdering every planet he can get his hands on since Gamora was what...six?
This is also on top of Ronan, who was explicitly in Kree space doing the same thing because he was some kind of religious fanatic. no one around to stop him.
The Ravagers, The Guardians, Thanos and his crew, The Grandmaster, and Ego all seem to have starships perfectly capable of getting from earth to wherever the hell they want in a matter of hours or days. The slowest I've seen was a trip quoted to take "a couple of months" to go from Sakaar to Asgard if they didn't use a wormhole. Where has CM been traveling to for 25 years? She could have gone back and forth to earth a thousand times over by caling a Space Uber.
How would Fury learn anything? SHIELD has been dismantled since Winter Soldier. He has no real networks or influence anymore. And the only way he can think of to reach out to Captain Marvel is a 90s era pager with some extra stuff glued to it. Those things are basically one way communication.
That theory has a ton of holes in it, man.
