The human element.
"He spent all day in a cafe."
"He spent all day in a cage."
Spelling checker says both sentences are okay; it can't notice the typographical error caused by a misaligned finger stroke. Grammar checker says both sentences are okay; the nouns are in the right place, all tenses are correct and there's no plurality problems. Even an AI program will not see an error, because it can not actually understand context.
Only a human being is capable of reading the sample sentences and wondering what is going on. The obvious answer leaps to mind, and we instantly deduce the mistake. We look for clues in the prior and succeeding text to understand context and determine which is the correct sentence. Or if there is nothing else to the writing, we go for the logical explanation based on our truly large language model of experience.
And then our sense of humour cuts in and we think "Well maybe it is the other one; you never know what people will get up to!"
To all the pedants out there who will find the mistakes I put in this post and think they're very clever for doing so I have two words, and they ain't "well done"!