there's lots of little reasons for that really
-most battles are fought on battlefields (also in reall life), these are simply places that are strategically important and convenient for fighting pitched battles.
in other words no one cares about the little rocky peninsula at the edge of the island so no battles are fought there but the wide open field in front of the islands main town is a place worth defending.
in a wartorn place like the old world that means that on certain acient battlefields there's generations of warriors lying in the ground, either hurriedly buried in a soldiers grave or left where they died as their buddies legged it off the field.
necromancers and other undead have a natural affinity for death, walking on a battlefield that has so many generations of corpses buried in the soil would be like walking in a rich field of corn for them, they see the potential everywhere around for them to raise.
additionally, provided the magic is strong enough necromancy doesn't even need a corpse, when nagash was destroyed his body was obliterated in to miniscule fragments of black tar like material, the very fabric of death so to speak.
his necromancy was so powerfull that over a timespan of centuries all those countless fragments slowly traveled and dripped into his coffin reconstructing his body untill he was ready to rise again.
nagash is a powerfull sentient being that took a lot of resurrecting, a skeleton is a mere automaton.
it's possible that very powerfull necromancers are capable of constructing skeleton servants from the ether by drawing in ashes, fragments of dead bodies and pure magic as long as death is strong around them. (don't forget that even a mere level 1 battle wizard is a powerfull mage compared to the many magic users too weak for the battlefield, a battle necromancer is probably a lot more powerfull than your average graveyard skulker)
concerning wights, traditionally a wight is the corpse of a important person that has been enchanted into undeath so he can protect his own tomb.
graverobbers expecting to find a dead king with a lot of fancy jewelry find a wight decked out in his acient battle armour ready to defend his tomb.
necromancers don't actually create wights themselfs, they find barrows and grave mounds allready containing a wight and then use dark magic to bind the wight to their will. (or in some cases they don't even magically bind the wight to their will but strike a deal with them to find powerfull allies, lord Krell was a particularly powerfull wight, a chaos lord in life that struck a deal with the lichemaster kemmler and became his ally)
in essense, skeletons are easy to raise, mindless servants but wights are sentient undead, usually allready raised before the necromancer get's to them.
this is also why wights are much more elite, they are malevolent, sentient beings that can think on their feet and fight with all the skill they had in life unlike skeletons. (also why necromancers can't raise them, the magic that animates wights is not theirs, probably much older than their magic and possibly much more advanced or they'd have a army of wights instead of skeletons)