A mech like R2D2 was designed for repair work. For such a droid there is a question of the need for more then one direction view. That said it actually has a extendable auxiliary visual imaging system and a life form scanner. It also have the universal computer interface arm which can can be coupled directly into for example the system of a starfighter – which in turn has its own sensors etc.
One thing is the need for a 360 vision, another is the added complexity and use of resources, thus also power. The data received by a increased vision specter would need to be defended in light of this. All the data has to be processed, analyzed etc. and a decision made upon the gathered data.
For attack / surveillance robots etc. a increased vision could more easily be defended, but then again several has other sensory systems beside vision to add to the capabilities of data gathering / detection. One have several examples of robots that has no human resemblance and 360, or there about, vision. (Flying surveillance "balls" for example.) Guess it often also can be easy to overlook non-androids as being robots.
Sometimes they inhabit multi presence and are in the grey zone of what can be called robots: for example smart houses with AIs that often do tasks as well as having broad visual observation.
All in all the humanoid aspect is a big factor in regards to why some only have a human like vision specter. Then there are also, to get back to R2D2, aspect of the purpose of the (an)droid. Quite a few has been built to coexist with humans and the AI aspect has been the main focus rather then vision.
And again: as for R2D2, and with several other androids, robots etc. they have other means of “seeing” – or more correctly – methods of gathering data, such as various scanners and sensory systems etc. that extend the picture.
As with us: we have sight, but also hearing, touch, smell etc. that combined is processed to “decipher” our surroundings.