Because checkpoints are oh so horrible....the game isn't open world enough to bother with manual saving. Its basically at its most expansive a 2 corridor shooter in the SP (not a bad one though), the points that you would normally 'save' at are pretty much done for you. Also the time between engagements is ALOT shorter than crysis1, where you could spend 10minutes sneaking around an enemy camp, save, then happily wreak destruction and safe in the knowledge that if a truck does land on you, your sneaking is done.
Crysis 2 doesn't need that, you barely stop firing/punching/sliding/stomping robosquids and a pmc comprised of the dumbest people the world has ever known (i mean it made sense for the KPA to wan't your head on a spike, but New York is under attack, pretty sure most people would go "**** the guy who's systematically murdering his way through several battalions, lets kill the monster robots blowing our starbucks up".