Why is Instagram so messy?

I'm a photography enthusiast. For photo sharing, I use 3 services: Instagram, Flickr, and 500px.

Instagram is the default one. Practically everything goes there (cellphone images included). Flickr and 500px are for "thoughtful" images taken with real cameras (sometimes digital, sometimes film).

My unscientific observation shows that Instagram is the most messy of them. People post virtually everything there: a beautiful Swiss mountainery scene, random people passing by on the street, boring selfies with boyfriend, cute cat photos, workout pics, etc etc. Some people even set up their online shop on Instagram!

Seems like Flickr/500px users have different mindset, that is to publish proper, thoughtful photos. You won't find celebrities post their vacation snaps there.

My question is: why?