These plazas are commonly located in the old town, so a lot of inhabitants might have inherited the properties. Also, it's sometimes hard to judge how loud an area is really going to be before signing the contract - especially the night-life.
I recently rented a new apartment in a crowded street and while it was certainly a mistake to move here, I didn't expect it to be _that_ loud since when we were there for the apartment viewings there were also a lot of other interested parties and it was distracting from the street noise.
I really wish there would be a project to distribute those noise-tracking devices and make the realtime data available to the public. It would definitely influence buying/renting decisions and help for city planning as I think this serious problem is only tackled on the surface and it brings down quality of living.