Although an element of prestige was obviously there, they went high also for the usual reason: cost. Italian cities like Firenze and Bologna, at the time, were

Although an element of prestige was obviously there, they went high also for the usual reason: cost. Italian cities like Firenze and Bologna, at the time, were among the richest in the world, and real-estate costs were sky-high. Buying land to erect some wall or other fortification, in the middle of the city, would have been a huge waste of money. Even roman popes, when they decided they wanted a fortified structure, just reused the roman-era mausoleum of Hadrian - building from scratch would have been prohibitively costly.