I've done that and physically moved them.I did encounter a woman who was most reluctant to move her bag a few weeks ago on the tube
I asked her why she imagined for one moment that I wanted to Stand
So they do exist but not often
some people have been known to threaten to sit on any bags which results in them being moved pretty sharpish
Despite a user name of need help quick, the op has, it appears not returned to read our collective words of wisdom.