I don't understand Diablo III

What are you playing it on ? PC ? Console ?

Are you going for online or solo mode ? Must make sure you pick the right stuffs in the menus..

Also it's a Blizzard game so you absolutely need to buy the lastest expansion or not only you won't have the full game but you will be pretty much penalized for not having paid more.

Pretty much everybody is lvl 70 and doing (G)Rifts/Bounties on T6 or playing Seasons (you will need the expansion Reaper of Souls if you dont already own it).

you don't see real people in campaign before vast majority of them bought game long time ago went through campaign already and in case they want a new character they will rather have their friends boost them to max level or level up in adventure mode because it is faster and they already know campaign anyway so no need to replay it.

Game is very far from being dead, but everyone is focusing on playing Seasons and/or EndGame - if you look for max level high torment Nephalem Rifts for example you will see hundreds of online players for example :>

unless you invite friends to your game or join others, it's single player.

also, the game actually begins once you hit the lv cap (d3 = lv60 and lv70 for reaper of souls), that's when you start to farm gear and try out builds.

Mot many people do the campaign anymore. Everyone is busy in Reaper of Souls, doing rifts.

Ok guys, thank you very much for your answers!

As others have said without RoS ( Reaper of Souls ) the game is pretty much dead.

Also right now the PTR is very busy as a huge new patch is about to come out.

If you need anyone to hop on and show you the ropes, shoot me a message I am always willing to help out Seasonal or Non Seasonal toons from the SG community.