I'm guessing user error or camera.
Short version : expecting too much.
Example image from a Canon 6D 50mm Canon f/1.8 lens, ISO320, f/3.2 1/200.
Is this out of focus ? Look reasonable to me.
Using back button focus, seems to focus fine and then I look at the image, zoom in and it's terrible.
What in the world could be the problem?
Zooming in too much, I suspect.
I would think a shutter speed of 1/200 for a 50mm shouldn't produce camera shake?
The usual rule (1/focal length) as a shutter speed to avoid shake was designed for approximately 8x10 prints hand held when viewed.
If you're zooming in on a computer or with the review function on the camera, you're going way beyond the intended use of this rule.
Zooming in is the equivalent of making a huge print and viewing it from up close. If that's the standard you aim for then it will require considerable effort in terms of technique, not just a high shutter speed.
Think of that shot in terms of how you intend to use it. You're almost certainly not going to need to make a huge print and viewed it from a couple of feet away. You're going to make a small print or view it (as a whole, not zoomed in) on a smallish screen. At that level it's a perfectly good, sharp image.
Also consider that just a little careful sharpening in post processing with any editor (GIMP,Lightzone, etc.) will greatly enhance the apparent sharpness. And you can adjust that to taste.
So again I'd say it's a perfectly reasonably focused image for a normal use. It might not be quite up to the standard of the cover of vogue, but then you'd need to spend an entire day nailing that kind of shot, and typically you'd be an expert shooter to do it using every technical aid you can.
It seems to be hit and miss. 3 shots after this one was perfectly focused.
This comment is interesting as it suggests a technique issue, possibly aggravated by the need to zoom in too close on screen, is the issue. It's not definitive, but 1 in 3 would be a typical success rate related to shake.
Note Michael Clark's remarks in his answer in this regard.
Also, just in case, note that if you're framing the shot with the screen, rather than a viewfinder, you're going to make shake a bigger issue. DSLRs are designed to be held in a particular way using the viewfinder to minimize shake.