I suspect that the manufacturers are frightened that someone will undercut them if they conform to an open standard.
Are you sure? it is really hard to predict what would have happened. Without the clones would the Apple II have taken off (it did have clones, but they would have needed to come up with a 16/32 bit upgrade pretty soon - the IIgs comes to mind though it was too late) , or maybe the mac?
Sure IBM had a name and would have got some sales, but would the platform take off? IBM did make a ton of money on PCs even if they were a bit player overall.
We can never know the above.