Because the gadgets are not strictly part of Jira. You can use them in other applications. To support that, they have to have their own code for some things, and inherit as little as possible from the application they are running in.
If you want to change gadget styles, you either need to work out where they're getting their styles from the main application (if they are at all) and override that, or, more likely, take the plugin apart and modify their styles.
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