Stack Exchange doesn't want comments that just say "thanks".
They clutter up the format & turn it into a forum.
It is a Question & Answer site, where each response is intended to be an answer to the question raised, in an overall attempt to promote learning. Comments may be used to provide &/or request clarification, but are not intended to promote conversation.
Comments that are simply "thanks" are very likely to be deleted.
Comments are also liable to be tidied up, erased, once a suitable answer has been found & so should be considered ephemeral & not needed in order to fully understand the question & its accepted answer.
Instead, the correct way to 'thank' the author of a post is to upvote it.
The minimum reputation requirement is to ensure people have sufficient time to learn the basic concept before being able to participate fully, higher reputation leads to greater privileges.
You can find this & much additional useful information in the Help Pages