Whilst the are many very long domain names, there are very few short domain names.
These one-letter domain names were registered before the naming authorities introduced rules.
There are a total of only 736 (26 x 26) alpha 2-letter .com domain names possible, or 1188 (36 x 36) alpha-numeric 2-letter .com domain names if you include the 10 numerics 0 - 9 to the 26 letters of the alphabet a - z.
With newer suffices such as .biz, .info, .us etc then it is not possible to register 2 letter domain names.
see .BIZ List of Reserved TLD Strings for examples of typical retrictions
For country codes the situation is generally similar. Until recently with Nominet naming rules 2 alphabetic-character-only domains (e.g. az.co.uk) were not permissible while 2 letter alpha-numeric domain names (e.g.: z9.co.uk) were allowed. Some 2 alphabetic-character-only domain names (bt.co.uk, it.co.uk) were registered before those rules were introduced and the rule was abolished in 2011.
For some example of very long domain names see here.
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