Welcome. I'll try to address your question:

So, why do people call it spam when you ask them to follow you ?

Well, imagine you write a post on economics. Its taken you several hours, and you eagerly await the opinion of those whove read your post.

And then you get THESE:

"Great post. Follow for follow."
"Nice, follow me back"
"Funny! Pls follow"

• First of all, it doesnt seem like any of these people have even read your post.

If you want to be taken seriously by an author, maybe even have them check out your stuff, you need to start a conversation.

And the only way to do that is to interact with the author. If you have similar content in your posts, dont just say so.
Begin comparing your opinions, arguing a point, or providing your own evidence for one of their points !

• finally, when we other authors see such comments, we kind of imagine this was what went through your head:

"Oh.. (s)he seems popular... maybe if I hang out with him/her I'll be popular too?!?"

Following someone is supposed to indicate interest in the content they are sharing. Its not supposed to be the ridiculous Facebook Friend-em-all game some people seem to take it as !!!

After all, if you follow 100 people, a lot of work would be required to read and interact with their possibly daily posts... and thats without taking into account those with 4+ posts a day !!