Now days as a high level everyone takes the game seriously and it makes things not as enjoyable
This is true but has it occurred to you that it is because they take the game seriously that they made it to end-game.
The game after all is just a grind with a focus on killing mobs to get xp. That just simply isn't fun to do all the time, especially with largely diminishing returns. So it's at this point that you havs to choose between grinding for xp and enjoying the game for the little content it has outside of that.
Plus, do you think players who are willing to spend a lot of money on the game for elixirs are going to waste it on having fun with everyone else? Or do you think they are going to spend that time using their platinum on trying to get an edge on everyone else via levelling?
All the bosses worth killing require that end-game mentality even at mid-level because they're difficult and there is very little to no reward for killing a single boss once. The exceptions to this rule are high level raid bosses such as Mordris and Hrungner. But you can't even kill them without the rewards of killing other countless bosses a countless number of times. Also, just try to take an event boss from a clan full of serious players; they have lower levelled alternate characters who can outlock you too if they wanted.
So in conclusion, those that have the kind of fun you're talking about are the low levelled players who hang around in the castle. The players who take the game seriously are the ones rewarded for it.