When hardware products have nonintuitive limitations like this, it's often because at the time the product was designed, there was no good SOC (System-on-a-chip; a single chip with CPU, I/O, and possibly RAM and FlashROM) that met all of the design goals, and no good way to supplement a "close enough" SOC with external chips and still meet the design goals for cost, power, thermal, size, etc. It could be that the only SOCs at that time that had GigE took too much power or dissipated too much heat or plain cost too much to build a successful product around.
Although this particular limitation might mean it's not an ideal product for your situation, I'm not sure it's that big of a deal for most consumers.
Since AirPort Express is one-band-at-a-time, most people are going to put it in 2.4GHz so that their 2.4GHz-only equipment like their iPhones and iPod touches and legacy hardware can still connect. Note here that Apple limits all their 802.11n gear to only use 20MHz-wide channels in 2.4GHz, in order to "be a good neighbor" and leave room in the band for other 2.4GHz devices to use, such as Bluetooth. So a 2x2:2 (2 transmit radios, 2 receive radios, capable of 2 spacial streams) system like the AirPort Express, limited to 20MHz channels, can only get a 144.4 megabits/sec maximum signaling rate. Because wireless Ethernet has a lot more overhead than wired Ethernet, the maximum TCP throughput you could get would be around 72-80 megabits/sec, which 100BASE-T can handle just fine.
There's also an argument to be made that most consumers don't have high-bandwidth needs within their own homes, and care mostly about getting to the Internet, and most home broadband connections are slower than 100 megabits/sec anyway.
