Couldn't it be like a radio antenna (wifi is radio after all), telescopic and without any encasing?
I believe the main reason is ESD protection. Many of these antennas are connected directly to RF chips which are quite ESD sensitive. This is especially true of the GPS antennas. For radio antennas (FM/AM) it is much less of an issue, so there are plenty of bare antennas.
The other two reasons are structural support and protection against detuning. While encasing the antenna in plastic does change the resonant frequency a bit, the antenna is designed together with the cover. The resultant antenna is less vulnerable to detuning (ie someone touching it, etc) than the bare wire.
For both cost and performance reasons.
A plain copper wire covered in plastic is the most efficient implementation for an antenna. Copper has good conductivity and the plastic keeps it from oxidizing. Also, since the plastic provides the physical support, the wire can be relatively thin, minimizing the amount of copper required.
A bare-metal antenna needs to be made from other materials so that it doesn't corrode. These metals have higher resistivity than copper, and the extra steps required to do the forming and plating add to the production cost. And since it needs to be self-supporting, more metal overall is required.
A WiFi antenna doesn't need to be collapsible because its full length is just a few inches anyway.
Antenna technology is quite different for higher frequency radios. FM radio is approximately 100 MHz while wifi is 2.4 GHz. 24 times the frequency means the antennas need to be 1/24 the size. Also, wifi antennas are designed to be as compact as possible while remaining omnidirectional with decent performance. Generally they use PIFAs for wifi in laptops and cell phones. PIFA stands for Planar Inverted F Antenna. This page has a description of how they work: http://www.antenna-theory.com/antennas/patches/pifa.php . Basically, it's a technique for making the antenna very small and still have it work at the proper frequency. As PIFA antennas are fabricated on fiberglass circuit boards, they are a bit delicate and they need to be put in a box to prevent damage. Also, antenna characteristics can be changed if a foreign object comes into close proximity with the antenna. Anything in the near field can affect an antenna's performance. Remember all the problems Apple had with reception on one of their new iphones a while back? Touching the antenna with your finger will degrade its performance, affecting the ability to send or recieve a signal.
As for why you never see telescoping antennas on wifi devices; wifi antennas are so short that making them telescoping is pointless. Wifi operates on 2.4 and 5 GHz. 1 GHz has a wavelength of 30 cm. A quarter wave antenna at 2.4 GHz has a length of 30 / (4 * 2.4) = 3.125 cm. A half wave antenna would be double that. And a 5 GHz antenna would be half of that. Quarter wave antennas are advantageous for several reasons. First, they can be driven from one end instead of from the middle and they have a relatively low impedance. Second, they're small. Third, they have low gain and therefore are not directional (e.g. you don't need to point it at the wifi router to get online).
Never underestimate the voltage at the end of an antenna - this where it's impedance is highest and some antennas are known to generate 1000's of volts. I'm sure that on a walkie talkie or WiFi antenna the voltage will only be tens of volts peak so don't worry too much.
Plastic = insulator for protection (maybe)