Why do you have to buy it for custom levels?

Paying money makes me sad  
And this game is fun but not as environmental and good looking like the good o'-fashion Knytt. Srsly, why should i buy it?

If you like the game and/or want to support Nifflas, buy it. If not, don't. There are no magic words that could be said to make you want to buy it (a fact that advertisers haven't yet realised), so if you don't think it's worth $4, don't pay.

Srsly, why should i buy it?

Well, that is a good question!
First, we need to evaluate how much you are willing to spend to support Nifflas game making, and how much you feel that the feature of user created content will be worth to you.
Then we need to compare the result of that evaluation of the actual prize of the feature in question.
If the amount of money that you are willing to spend exceeds the prize, then you will have made a good investment that will in turn increase your enjoyment of the game.

Nifflas can't earn a living making only freeware games...

...this game is fun but not as environmental and good looking like the good o'-fashion Knytt. Srsly, why should i buy it?

If you want the game to look different, why not pay the $4 for custom level capability and make your own level that looks exactly the way you want?  The simple graphics in the main game are a deliberate design choice by Nifflas, but there's no reason you couldn't make a level with graphics that look more like Knytt or Knytt Stories.  If you think the game if fun but you want it to look different, that sounds like a perfect reason to make your own custom content. 

Also, how do you know if the third-party levels that already exist look good or not until you pay the $4 so you can try them?

Dude, it's $4... FOUR DOLLARS... I spent more on coffee today