Hello Kazuaki
The reason for the number of channels is simply the definition of the standard itself, the available range of frequencies in the specific band, and the width of each individual channel. When it was conceived, 11, 13, or 14 channels (depending on where in the world you are) of which only 3 are non-overlapping, was considered enough for the needs of the day. For this reason, the specific frequencies were defined, and the specific width of each channel was defined.
Technically, it is possible yes, but in order to do so, you must change the standard. The standard however has been set, and all manufacturers comply with that standard. If you create smaller channels, both end devices and access points must support this. There are no plans to change this standard at the present time.
Even so, making the channel widths smaller will cause a decrease in available bandwidth, since from a physics standpoint, the smaller the width of the channel, the less bandwidth it can handle. However, the width of the channel is not the only thing that can affect the bandwidth, but it is just one of the fundamental parameters that defines it.
It is true that higher frequencies are capable of carrying higher data rates, but again, the frequency is not the only characteristic that affects bandwidth. The methodology used to place data on the carrier wave is also involved in this. Each standard is designed to provide specific capabilities at specific frequencies.
The main purpose of 802.11a was to provide a wireless service in a “less crowded” frequency range of 5GHz. It was the first to use Orthogonal Frequency Division Multiplexing (OFDM) which gave it the capabilities of up to 54 Mbps. Several years later, OFDM was used to define the 802.11g standard which yields the same speed. So you see in this case, the use of OFDM was the defining factor as far as the available speeds go.
Physically speaking, higher frequencies can carry higher data rates, but this is only true if the methodologies with which data is placed on the carrier waves takes advantage of this physical capability.
I hope this has been helpful!
Laz