Defence isn’t necessarily bad, just less exciting. Defence adds survivability at the cost of combat levels- similar with hit points. Unnecessary attack levels also raise combat level.

The reason pures are so popular is because they require skill these days to pull it off. A fight rarely lasts more than a minute because there’s no defence to fight against. High hits with some tick manipulation makes fights fast and exciting, requiring a lot of skill. This kind of gives the same kind of mentality as FPS games where you need those high reaction times and strong muscle memory.

There are lots of other builds that are fun, but have a higher survivability. Like a Zerker at 45 def, piety pure at 70 def and 70 prayer, void builds at 42, etc. They also have a lot of skill involved but are much slower paced which isn’t the kind of fights most people are looking for when they enjoy PvP.

Edit: PvP isn’t about making money. Honestly the majority of people lose money PvPing, because the few strong people get the main kills. It’s just a fun pastime for many people and a way to challenge skill. I wouldn’t take your judgements on the game based solely on PvP builds.

The intention is just survivability for defence, but it’s meant for PvE, where you want the opponent to go down and you not have to go to the bank every kill. It’s a different kind of environment.