What about software spell$-$checkers makes them incomplete tools for proofreading?

Spell$-$checkers conform themselves to the way you typically write sentences and paragraphs, and they make mistakes if they are too familiar with your writing.

When spell$-$checkers check both grammar and spelling, they lose effectiveness in checking spelling.

Spell$-$checkers do not conform themselves to the way you typically write sentences and paragraphs.

Spell$-$checkers often fail to distinguish between homonyms, leaving errors in a document.