Taking a selfie requires no preparation. We've all got cameras on our phones, so it's easy enough to do. I'm not really a selfie person, per se, but in the right place and context, I might snap one for posterity.

A tourist walking around with their phone in a selfie stick has put preparation and forethought into the endeavor. Before leaving their hotel to see some wonder of the world, they consciously thought, "I'm going to take a bunch of selfies today so I can post them on social media and gain influence because I'm somebody." This attitude represents everything many people blame for giving rise to the modern overcrowded tourist destination experience.

I'm sure there exceptions, and some people just like pictures with themselves in them, but this is the impression it gives off. You asked why people hate selfie sticks, and this is a likely reason.