Why is a derailleur guard present only on more affordable bicycles? Looking at http://decathlon.co.uk, I see a derailleur guard only on more affordable bicycles, e.g. this one.



This one does not have it, and there seems to be no obvious way to install one.



Are derailleur guards important? Why does the more expensive bicycle (with higher component replacement costs if something breaks) not have one? Is it correct that one cannot be installed on the one shown in the picture? If it can be installed, is it recommended?

One might always fall, or a parked bicycle may get knocked over.
Derailleur guards are rarely found on more expensive bikes for several reasons:


Protection: These guards only protect from minor contacts. Forces that would bend a typical derailleur would also bend these guards. Also, more expensive bikes usually have exchangeable derailleur hangers that are supposed to be the first thing damaged in a crash (and, thus, prevent the more expensive derailleur from more damage).
Weight: if the bike is handled properly (as owners of more expensive bikes tend do), the little added protection against minor touches is generally seen as not necessary.
Style: These things are ugly!