[ithkuil]

Because a tracker could detect that your topic of interest are fake? How would they know?

[majormajor]

If they're relatively unique they can just track you across sites and use your browsing history instead.

[janc_]

Depends, but either the topics never changing, or them changing too randomly, could all be detected when combined with other tracking, and become part of your identity as such...

[sltkr]

Simple. If the Topics API is disabled, then you are a nerd who spends too much time on sites like Hacker News, and they can show you ads accordingly.