Well maybe you are right, I havent played many MMO's just this(Which isnt really an MMO) BDO(Which lacks any challenge outside of PvP) FFXI, FFXIV and a few others but not for a very long period of time.
On a pure gameplay front,, I would say I enjoy this game more then FFXIV but I would say the Raid Boss encounters in FFXIV are more enjoyable,Especially on Extreme, Savage and Ultimate(Everything outside of that is just boring though) I find the mechanics are just better thought out and give the battles more depth and team coordination(FFXI also works in a similar manner), which is what I enjoy when playing an MMO,)If I want solo challenge, I just play SoulsBorne type games Or Monster Hunter.) Here it kinda just encourages lonewolfing and the mechanics itself are incredibly shallow, The thing that makes it worth while is the combat and character building which has more depth then FFXIV shallow progression system..
Yeah I have heard "solo" runs and self imposed restrictions are usually the way to go for this game if you want challenge, but I find self imposed challenge(Handicapping yourself) not really my thing, I am more interested in how the game can challenge me, not how I can cripple myself to make the game more challenging. But I understand this game isn't really about that so it doesn't really bother me in the long run, its still fun in most areas.