The siege of Magdeburg in the 30 years war killed 25,000 of it’s 30,000 citizens (Atrocities, Matthew White). And left it so devastated that “the city burned for three days”, with “the living drawling from under the dead” and “infants sucking the breasts of their lifeless mothers”. The survivors were mainly women who were kept by the mercenaries who besieged the city. Today Magdeburg has a population of 230,000. Why wasn’t Magdeburg, and many other destroyed cities simply abandoned?
Apart from any sentimental value attributable, a city is more than many people living in one spot. Cities, especially ones with long histories, were usually built in a strategic location of sorts. Thay could either be a military one or, perhaps more commonly, an economic along major trade routes or crossroads. The income or control this location offers is usually worth rebuilding the city.
(Also: Even though a city might be destroyed, many building materials and some infrastructure (roads, sewers, foundations) remain reusable, thus facilitating reconstruction.)