Apple Pages has does not help you're grammar

If you are concerned about English grammar, then use Grammarly (they also have a Mac application) with Pages. Why questions for Apple can be sent to the Pages : Provide Pages Feedback menu item, as we are fellow users with no more knowledge of Apple internal business/product decisions than you.

In the case of your post title, I suggest that you enter your and you're in the Apple Dictionary app, where you have set the top item in its Preferences to New Oxford American Dictionary (American English), and read about the usage for the possessive determiner, and contraction respectively.

The red underline indicates this is being flagged as a (possible) spelling error, which would indicate that "your" is missing from the spelling dictionary—which is odd in itself. Tell spellcheck to add that word to the dictionary, and the red line won't be seen again.

I'm assuming your choice of word in the topic line was intended.

"Why has Apple not fixed Pages' grammar?"

A question whose discussion is beyond the terms of use for these user-to-user communities. The people who determine Apple's priorities don't participate here. You'll have to direct the 'Why" question to Apple.

For what it's worth, Pages 5.6.2 did not flag "Your", but did question Physio/Aide (as did the spill chucker that monitors the mistyping here on ASC).

Regards,

Barry

Venting, ranting, and surveys of what others may think are outside of the terms of use for the Apple Support communities.