are you telling me that in 2016 when nasa are sending buggies to mars and probes to the edge of our solar system.that they cant be bothered to turn the camera around and take a real full shot of earth..
A) Hardware designed for a particular set of mission parameters at the target may not be suitable for Earth observation. By way of analogy, consider medical X-Ray equipment. An X-ray is an X-ray is an X-ray, right? However, the X-ray machine your dentist uses to look for cavities is quite different from the one they have at the ER to look at your broken arm.
B) Mission profile may not allow for spacecraft orientation appropriate for Earth observation. Spacecraft have a limited power budget, and to conserve that power they often do not power up the science instruments until the target is near. In many cases the spacecraft would have to change orientation to bring a camera to bear on Earth, which expends propellant or power on a task not related to the primary mission.
C) Some probes actually have done what you suggest.
A lot of probes, in fact. Sometimes the objections above are noted and overcome by mission planners and we get a photo of Earth during a mission to somewhere else. Messenger did it on its way to Mercury, as did JAXA on its way to Venus. The Lunar Reconnaissance Orbiter and the Mars Reconnaissance Orbiter have both done this from their final destinations. Rosetta took Earth crescent photos on multiple gravity-assist passes of Earth on its way to the comet it observed (crescent because Rosetta was approaching Earth from higher orbit, which put it on the night side of the planet). At least two Jupiter probes (Galileo and Juno) have taken Earth photos as they flew past us for gravity assist maneuvers. Same for outer planet missions like Cassini (Saturn) and New Horizons (Pluto and Kuiper Belt Object 2014 MU69)
In fact, the more I look, the more it seems that options A and B are the exception and option C is the rule.