Many publishers prohibit the author from uploading the published version of the paper by the reason of the copyright but allow the author to upload the post-print (the last version of the manuscript) or the paper as included in the collected works of the author. So, I'm uploading my papers mostly in these forms.
I agree with all the previous answers and add that some authors prefer to give the hyperlink for an article in open access that it's uploaded. Because it increases the number of readers in the Journal platform.
May be fear of copyright law for most of the closed access journals. Some journals request for as high as US30 before you can access a copy so uploading it on RG platform infringes on their rights as closed access journals.
Apart of issues of copyright reserved, some researchers do not like giving their full text for reasons best known to them. When you request for full text of such research items for may have to wait till eternity because they will not respond.
I think that the idea of belonging to a community like this, is to share knowledge, so the ideal is to have easy access to it, at least all my publications are public
For many reasons: maybe due to copyright from journals, perhaps their future work has to do with that, maybe they love it so much they do not want to share, maybe they do not have it any more, etc.
This is sadly true, but there is nothing more rewarding than public exposure. By exposing ourselves to public scrutiny and criticism, we can improve our academic stance.
When you signed the copyright from with any Journal or IEEE conference which are not open access, you are no more the owner of your paper. So that is the reason why the are not public. The is the rule we had to obey. You can get it in personnel correspondence with the author.