Simply because the Zeiss people have tried their hand at taking macro and close-up pictures. And they have found out what happens when you set an AF mechanism to work close-to.
What happens is a perfect storm of confusion - right there in your hand.
You can confirm this is you have a macro lens that has an AF mechanism incorporated into it. Several of the makers have done this , both in the DSLR and mirror-less divisions. The lenses that feature AF can be extremely good performers, but in certain circumstances you will never discover it. What you
will learn is a whole new vocabulary...
Start out with some basic factors:
a. Depth of field at close-up distances is limited. At macro distances even more so. You can't dodge it, it is a fact of optical life. You can ameliorate the worst by using small apertures, increased illumination, and shorter focal lengths, but the truth is the world is a deeper place than macro lenses can see.
b. Close up subjects tend to bounce around in the viewfinders. Even if they are blind worms or dead specimens, they still bounce. Do not blame the worms - it is you. Everyone of us has more tremor in our hands than the macro world can stand. If we are trying to avoid the use of a tripod or studio stand we are in shakey-land.
c. Shakey-land and autofocus do not mix. As you try to focus with the AF, it starts to read your inadvertent to and fro movements and to compensate for them. The subject swims in and out of focus - in some cases it swims in and out of view, the DOF is that shallow. Every movement exaggerates the work the lens has to do and in the end some cameras just give it up and fire off anyway - to have it over and done with. Result - out of focus shots.
The way to defeat the attempt by the lens to do its job in spite of you is to do its job for it. Switch to manual focusing on the camera body...or buy a Zeiss Makro lens...and set the magnification on the lens barrel to what you want it to be.
Then slowly push the lens and camera toward the subject as you watch the LCD screen, the focusing screen, or a suitable EVF. When it is focused, fire away, with no delay and as little motion as you can manage. Your percentage of in-focus hits will rise dramatically.
If you are shooting with a Zeiss Makro those in-focus shots will be extremely crisp and free of distortion - true scientific records of whatever you have seen. You'll still need all the tricks of the macro studio to get light down onto the subject at the correct angle and with sufficient intensity, but that is another topic.
Also a topic for the future - mechanical aids to that focus.
