Why do we play Maplestory?

While I agree in part with your statement I want to just play devil's advocate and say that on the other side of things there could be those people who just don't enjoy leveling after a certain point and would just rather PQ with others for fun, or level up their trait skills/make items to sell, or just be a mercher. My point being is that yeah the main idea of the game is that you level up and increase your skills but again that's only 1 aspect to the game.

Though I have to say I am one of those people who doesn't see the point if you aren't going to aim for a high level and good gear...but again, just my mindset and I respect other peoples' choices in how they play the game.

Sure, I'm fine with devil's advocate.
PQs have their level restrictions. Restricting yourself to a certain level restricts you to certain PQs.
Levelling up traits requires... uh well I guess charm can only effectively be increased through NX, and ambition through bossing?
As for making items to sell... well, most worthwhile craftable items are easier and cheaper to get from boss drops than to actually spend time and effort collecting the ingredients to make them (this of course is an issue that should be addressed and rectified by the developers)
And I'm not even going to deal with the crazy crap that deals with merchanting here. If you insist we can take this elsewhere.

I guess, ultimately, while it's "only one aspect of the game", it helps you with those other aspects of the game.