That's one guess. Here's another:

https://www.sciencing.com/1810624/why-pigeon-nests-look-horrible/

There is an old adage that says, "simplicity is the ultimate sophistication," and pigeon nests are the perfect example of this philosophy in action. Even before humans urbanized much of the world, pigeons were never big nest builders; they utilized natural roosts, favoring pockets in the sides of cliffs. With the rocks providing ample area to roost and incubate eggs, all the pigeons needed to do was to place a couple of twigs around the egg to keep it from rolling away. As cities grew in the wake of industrialization, pigeons adapted to use concrete structures in place of these natural cliff roosts, still sparing them the effort of building a complex nest.