Different people have different reasons, some feel you're an ideal exchange partner but are already talking to others and have no time, so they decide to keep you on their list maybe for future but get side tracked (i've been guilty of this).. some people are new, some are busy, some are picky, some overwhelmed with messages, some just outright weird, hence why I've learned not to take it too personally.
I guess it's not as straightforward as "you know the language i'm learning, I know the language you're learning, let's exchange"... in reality it's not always easy to meet people who click with you, language learning or not.

...they are just "friends" collectors.

Maybe they are busy ⊙▽⊙

I wouldn't say they are weird. It happens in every social networking website.They add you probably because they like one of your comments or maybe they don't know how that button works, it also could be they just want to be your friends. It wouldn't hurt to keep them in your friend list as long as they don't send your spam or harass you.

I consider it as a way to show you that you shouldn't waste your time of them :)