Because the Derplings will one day rule over everyone and we will have to hide in fear, so we must appease the Omega Derpling so he spares us.
Terraria Community Forums
The Storm
sharkman0101
I never really found Derplings to be that much of a nuisance to deal with
Dapling
The Derplore would have you believe differently... 
COOKl
It's kind of just a thing @Storm Diver_102 started because of the derplore, and now it's everywhere, even with my (shameless plug) thread Pixel Art - Derpify Me
Dapling
The Derplore was a thing before Storm Diver noticed it, he just accidentally helped it gain popularity.