Shorts is a growing format on YouTube. The inventory of videos is increasing over time. If someone clicks the X button indicating the recommended shorts are not relevant after 30 days there will be many more new videos. Potentially some of these new videos will now be relevant enough to show the user. Additionally the X button is not labelled unless you o haver over it after which it says "Not interested." If it were permanent people may accidently click this button and then they will never be recommended shorts even if they enjoy them. People's opinions about shorts way change over time. After a month they may actually want to engage with the format. Having to go search through some menu buried somewhere to enable shorts on the home page again is not user friendly.