It's a numbers count really on instagram. If its businesses they like to get many follow backs to make they company look good, so they tend to follow random. People in hopes they follow back automatically.
My insta is private for me and very few friends but I still get alot of business accounts which I've never heard or follow, sending me requests to follow which I ignore.
They you have fake profiles people make, to make them seem like legit followers then need these followers to follow other random accounts.
The ones that actually like your pictures are probably the ones that find you style interesting, other than that I don't know.
People follow insta account for various reasons, alot of food, travel design on insta, its not all about the sexy pics, people do like normal or simple nice pics.
I think its a similar issue on facebook, but I don't know, cause I don't have the follower option on and the only people on my Facebooks are family, friends only people I have met and talked to irl.