Why is a derailleur guard present only on more affordable bicycles?

Code of Federal Regulations Title 16: Commercial Practices PART 1512—REQUIREMENTS FOR BICYCLES Subpart A—Regulations

§1512.9 Requirements for protective guards.

(a) Chain guard. Bicycles having a single front sprocket and a single rear sprocket shall have a chain guard that shall cover the top strand of the chain and at least 90° of the perimeter where the drive chain contacts the drive sprocket as shown in figure 7. The chain guard shall extend rearward to a point at least 8 cm (3.2 in.) forward of the centerline of the rear axle. The minimum width of the top area of the chain guard shall be twice the width of the chain in that portion forward of the rear wheel rim. The rear part of the top area may be tapered. The minimum width at the rear of the guard shall be one-half the chain width. Such chain guard shall prevent a rod of 9.4 mm ( 3⁄8 in.) diameter and 76 mm (3.0 in.) length from entrapment between the upper junction of the chain and the sprocket when introduced from the chain side of the bicycle in any direction within 45° from a line normal to the sprocket.

(b) Derailleur guard. Derailleurs shall be guarded to prevent the drive chain from interfering with or stopping the rotation of the wheel through improper adjustments or damage.

One word KIDS, the little buggers love to jump off their bikes and let them crash to the floor, especially hard surfaces - these make perfect sense. I am about to buy these for both of my kids bikes. You can try to educate / plead with the ragamuffins or just pay the $10 and be done with it. Pick your battles ......

Not noted in the other, excellent answers:

Inexpensive bikes have a tendency to not be well cared for - they're purchased by people who either need inexpensive transportation or for kids (who are notorious, as a group, for not taking good care of expensive things*). More expensive bikes tend to be better cared for by riders who actually care about the bike they've purchased and would prefer not to have to replace unnecessarily damaged components.

When a casual rider of an inexpensive bike stops riding, he can often be seen simply dropping the bike to the ground and walking away*. If the bike happens to drop on the right side, the rear derailleur will hit the ground. Even from the relatively low height of a rear derailleur, damage can ensue. The derailleur protector can take significantly more hits than the derailleur itself can before damage build-up is enough to cause riding issues.

More serious riders of more expensive bikes rarely "drop" their bikes when they're done riding. The bike is carefully balanced against a wall/tree/rock/etc to ensure that the precious componentry and paint job are not damaged. If the bike is laid down flat, it will be laid on its left side precisely to avoid having either the front or rear derailleur come in contact with the ground.

*These statements based upon personal observation of my own children not heeding parental instructions to not drop the bike when finished riding, and personal observation of other people treating their BSOs this way. No offense intended to any casual rider who may happen to actually care for her bike.

Simple. They’re not AERO!!!!!!

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?

• Are derailleur guards important?
  Often a guard is on a bike geared toward kids or less experienced riders. If you are likely to crash (or "ghost ride" your bike) then a guard may help. If you don't crash then you don't need a guard.

• Why does the more expensive bicycle not have one?
  More expensive bikes are often marketed toward more experienced riders who - for the reasons listed in other answers - don't want the guard. It may be seen as not needed and added weight. Sometimes there is snobbery that prevents people from putting things like derailleur guards, kickstands, fenders, spoke protectors, reflectors, and other useful equipment on their bike.

• It is possible to install a guard on the one shown in the picture?
  It's possible to install a guard on any bike - even if it does not have bolt points - by using clamps as one option. It's always easier to install a part that is actually made to go on a bike but there are creative ways of attaching things to bikes.

• If it can be installed is it recommended?
  It depends on your situation. If you need a little added protection on your derailleur then I'd recommend it.

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!