Actually, many scenarios modify the game as much or more than any modpack. The difference in practice is that a modpack offers an alternative "epic" game starting from a settler and going thru long technological development, typically on a random map, whereas a scenario is focus on some single period, development, or war, plays in a specifc area, and usually only covers a comparably brief span of time. Colonization and peaceful expansion normally plays a much smaller role in scenarios than in mods or the epic game.