honestly, being a tank or healer takes a different type of person than just a dps. the reason people don't want to tank or heal is because they have the majority of responsibility of the party surviving on their shoulders. they do not want the responsibility nor do they want to be a leader. if a tank pulls mobs into the wrong location or turns them at the wrong time it's a wipe for the whole party. the more responsibility and advanced knowledge needed for a role in the party the less amount of people will be willing to do that role.

this is especially true with today's impatient, give it to me now, pew pew type of players. they want to do the bare minimum of effort needed to get things done as has been shown by people constantly complaining about things are too tough and should be lowered because their skill is quite bad. they don't want to put in any effort to improve their bad skills so they beg for the content that's already not hard to be made even easier.