I don't see why contaminated soil should be considered problematic at urban condo sites. Kids won't be playing in the dirt here.
It's more complex than that. A lot of it has to deal with groundwater and drainage. Redeveloping a site like that is bound to have lots of VOC related contaminants in the soil from gas/diesel reservoir and piping leakage and whatnot.
You do have a good point, a condo requires lots of excavation and will remove most of the soil anyway. However, the soil isn't usually fully recoverable with an urban site where your excavation is limited. It often means that drainage and water systems need to be worked around the remaining contamination, and vapour barriers need to be installed in the hole to contain any remaining hazards. Things like underground parking could possibly see an effect from contaminated soils as well, fumes and slow water leakage into a parking garage can bring VOCs and fuels into direct contact with people that way. Older gas stations only increase the risk as there is likely more contaminant.
It all seems kind of redundant but it's necessary to eliminate some bad "what if" scenarios. Nobody wants to be held liable if fumes or contaminated water start poisoning residents or affecting neighbouring properties.