People like playing DPS jobs and not playing tanks and healers. This is ingrained in players since the beginning of gaming. People generally do not like playing support roles. How many people want to play a goalie in football or a catcher in baseball? Defense in basketball? A military medic or radio operator? How many epic or glorious tales are there of radio operators or medics? No, films, novels, video games, anime are about wartime heroes: swordsmen, snipers, gun toters, samurai, people that kill and maim things in interesting ways. Support roles are integral to human civilization but they don't have the same limelight as more active and visible roles. Movies can't be made without cameramen and crew behind the scenes but they aren't visible. Healers and Tanks are the drummer/bass of the party, they carry and support but they aren't the lead singer/lead guitarist that get (most) of the fandom.

People (generally) want to be in a role that's visible, flashy, and visually impressive. Humans want their efforts to be be visible so they can feel validated. Gamers feel that healers, while important, aren't fun because their job is passive (to keep dps alive while dps kill the things with cool attacks). There is no way to change this mindset because it has been instilled in us. You can incentivize playing Healer or Tank with various rewards, but fewer people enjoy playing these roles because it's 1) not as flashy/cool, 2) more responsibility.