Yeah, one of the big problems with this game (imo) is that, from a certain moment in the development, there just weren't enough zombies anymore and it's never really recovered from that; the reason being that too many zombies are a big hit in performance.
There was a certain moment back in previous versions when it was pretty much perfect. Instant respawn of zombies in hub cities, which meant you could never clear them, and random wandering hordes composed of 20+ zombies. Sometimes, they would catch you off guard while looting a house, they would clog all exits and corners, and you would have to get lucky or get smart to survive. Those were the days.
Anyways, don't expect them to bring all that fun back. Pray that modding will take care of it.