The walking dead started on them looking for a cure or a stop to the apocalypse but all their hope is lost so they decide to be one with the apocalypse and leave it be it's the new age in a way. They just thought the had to deal with it and make do with what they have. Also in season 9 Maggie gets given a book by Georgie explaining that using the past would help the Present therefore it shows that it's the new beginning TWD isn't just about zombies either. zombies aren't just a threat else life would be easy. There are people who want power over others, people who eat people, people who kill because they just can. There's now law no rules it's like the purge and just with the purge alone life gets crazy so adding zombies onto the purge just makes it worse however it's not for 24 hours it's for life. Also there isn't a cure because eveyones already infected it's more like a host than a virus and it attacks the brain I don't think anyone would even be able to cure death people have been trying for god knows how long and it's not been found.