Why then do so many people insert a space between the words spell and check?
The reason is very simple. The word spellcheck is not included in the dictionary which comes with the software people use. When people type the word spellcheck the spellchecker tries to find suitable suggestions and since the words spell and check are both valid, a suggestion that is made is spell check with a space between the two words.
People across the world now believe the spelling is spell check, but in fact their word processor has misled them and they are now making a spelling error.
The preferred Australian English spelling files I provide corrects this and thousands of other spelling issues that occur when using Microsoft Office products. The same issues occur with other software, but least I can provide spelling files which correct the issues.