Because it's based on a %chance of events happening? I suppose you could come up with a complex formula to make the percent bonuses/penalties (from Luck/Misfortune or Order/Turmoil) meaningful, but it would seem easier to just use them multiple times.
On the other hand, the number of events does seem to scale with number of provinces.
On the gripping hand, it doesn't seem to scale at all linearly, which the check each province in turn would suggest.
I ran a test recently, after the discussion about events happening more frequently in low number provinces:
2 Nations on a 602 province map, provinces divided evenly. Both nations T3L3 Dom10. Temples in every province to push dominion up as fast as possible.
Code:
C'tis: provinces 1-301
Turn Events in province
5: 170
6: 68
7: 15,36
8: 172
9: 126
10: 3,84,126
11: 206,96,171
12: 65,283
13: 38,246
14: 280,4,167
15: 269,279,199,2
Midgard provinces 302-602
5: 541
6: 400
7: 326
8: 304,346
9: 570,590
10: 534,320
11: 566,399
12: 313,458,567
13: 458,442
I'm not sure what it proves, but it doesn't seem to be checking provinces in order. Nor does the number of events scale up as fast as I'd expect. I didn't test smaller numbers of provinces, but you get 2 events regularly and 3 occasionally with much smaller empires (30-40 provinces?) If each province was checked independently, until a cap was hit, I'd expect a 300 province maxed out luck/turmoil empire to be hitting the cap regularly.
I also have a hazy memory of one of the developers saying that events were driven by the capital scales, but I wasn't able to find the post again. It was a long time ago.