There are many good reasons for this state of affairs, but let's start with the biggest: marketing. Marketers always want to put their best foot forward in any situation, and specs are no exception. The fact that these numbers are essentially never achieved in reality are of little concern, as long as there is some basis in reality for the claim being made. Let's take the 150, 300, 450, and 600 Mbps, depending upon implementation, of 802.11n-based WLANs. These really are "real" numbers, but they represent the rate at which bits fly over the air between an access point and a client. Now, not all of those bits make it to the destination, necessitating error correction techniques and often retries of the transmission of a given packet. Some of those bits represent overhead in both the WLAN itself (Layer 2 and below) and network protocols (Layers 3 and 4). Radio waves naturally fade with the square of the distance between transmitter and receiver (i.e., exponentially), meaning that with greater range comes lessened reliability and thus lower throughput (this is often called rate-vs.-range performance, and is much more important than raw or theoretical numbers alone). Movement of an end point (i.e., a client in motion) represents a major challenge in terms of both the fading noted above and the need to correct relativistic changes in the frequency of radio waves due to motion, known as Doppler shift.

Note also that environmental changes can occur from moment to moment as well as simultaneously.

That we can build machines at all that work as well as they do is simply amazing, but we regardless can't fight the laws of physics. Add in differences between different environments (building construction, internal walls and floors, reflections from metallic objects, etc.), interference from nearby transmitters operating on the same frequency (particularly a concern in the unlicensed bands used by wireless LANs), network loading and congestion (based not just on the number of users, but also the instantaneous nature of their traffic and demands for service) and many other factors, and, well, "your mileage may vary" just scratches the surface in answering this question.

So one can't blame the marketers for being a wee bit exuberant, but also keep in mind that the engineers also have a lot to say about this, and product features, implementations, and performance do vary significantly -- much more than we find in the wired world. Thus given any particular set of conditions, expect to see significant differences in performance among products in the real world -- no matter what the specs might say.