Avoiding the argument over whether this game is designed mainly for PvP or PvE entirely and commenting on the time things take and whether 3 months is too long to wait.
Making video games is hard. We’re talking hundreds of people, thousands of man-hours, millions of dollars. If you think getting several million lines of code to all play nicely together is simple then I’m afraid you’re just wrong 
As for making a new level for PvE, it would not surprise me to find out that this would take upwards of 6 months from start to finish. From initial ideas, to concept art, to whiteboxing, to prettying, then adding in the scripted elements (dialogue, enemies spawning, doors opening when they should etc.), testing, bug fixing, design changes, implementing design changes, more testing and bug fixing, a few more tweaks here and there, more testing and bug fixing, going through certification (for X1 and PS4), potentially more bug fixing off the back of that. I just realised that I’ve failed to mention audio and balancing!
Making a level it not just one person placing some rocks and trees and hitting the “go” button, it is a MASSIVE team effort involving dozens of people for a single level. And get this, not everyone can work on that level at the same time! Of course, there are ways to help mitigate this. A level will usually be split into numerous “sub-levels”; there will be some for artists, several for scripters, audio levels, a few for lighting etc. often totalling well over a dozen. Then these all have to come together and play nicely too.
I’m still over simplifying this massively, and you’re more than welcome to feel that things are taking too long. I just hope I’ve been able to shed a little light on why things can take so long.
Apologies to the audio devs for not giving you enough love in the post! 