Because the parameters of AppIntents are limited.
Depending on how exactly you'd like this to work, you can have common places defined as parameters in your intent, and use that intent with its parameters as an App Shortcut. For example, you can define values in your app like home, work, and school, and you can have the user set the addresses for those items in the app. However, you're correct that you can't define an arbitrary String for input to an App Shortcut, such as for a dictated address or open-ended destination, like with your sample phrase.
If you need to take in an open-ended string as part of an App Shortcut and not just limit yourself to pre-defined places, then please file an enhancement request using Feedback Assistant so we can consider adding the necessary APIs for you to accomplish your goal.
Once you file the request, please post the FB number here.
If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?
— Ed Ford, DTS Engineer