Because they're not corpses. There's actually quite a bit of lore behind the game, but the basics there is "they didn't quite die" as the zombie type.
As for survival, stay mobile. Keep moving, keep scavenging, keep running away. My general gameplay previously was simple. Find the closest clothing store, sporting goods store, or military surplus store. Grab cargo clothes, preferably cargo pants and 2x utility vests, though some prefer a backpack that's your call, it basically prevents dedicated melee though. Then hit a liqueur store and a library. Actually, a library and liqueur store. Get the books into the small back room and drink ALL the booze (well, maybe like 6 shots). Wait a little for your drunkeness to pass and then spend time reading. Remember that mil. surplus store? Hopefully you have several MRE and bottles of water so you can sit tight and read. By the time you come out, you should be full, sated, have some crafty skills, and have a large exp pool. Then it's just a matter of using windows as chokepoints and moving quickly. Remember that you don't have to kill everything - in fact most times you'll be out of exp/morale and killing won't give you any skill!
Night has the same amount of enemies as daytime. The only difference is that zombies hunt by sight, sound, and smell, in that order. If you can see a zombie, it can see you. During day, that can be 2 map tiles away. At night, that can be 2 real tiles. You can have a pack of zombies across the street never notice you if you move quickly and smartly - even moreso if you can manage to get some night vision bionics, goggles, or mutations. If you're having trouble surviving, I STRONGLY suggest you go out at night instead. Navigate via the map to find buildings.
If you're playing with vehicles, as most of us will with the next merged update, then you'll mainly want to do the same thing, except find a bike or truck or something. Then keep moving. 20mph is fast enough to barely outrun a Fast, so you can easily run into an area, jump off, grab supplies, and take off again before anything catches you. You'll also want some mechanic skill and will love hardware stores, upgrading your bike to include some more boxes and armor will greatly expand your capabilities. Try not to ram zombies unless you've armor-plated your front - and remember that the roof is made of steel plates, so you can sacrifice weather protection to provide more bumper-power, if you've got a car or truck.