Many countries mainly use second level tlds, e.g. co.uk, gov.uk, co.th, com.au, .com.my, etc. because the registry of that country decided to make those available to the public, and the first level alone is not.

In some cases the first level tld subsequently became available to the public - for example in 2022 .au has become available, but the original second levels, e.g. .com.au or .gov.au remain more popular because the public is more familiar with it.

PS I just saw here that .uk also became available to the public in 2014 https://en.wikipedia.org/wiki/.uk