I think activity focused on topic uses the default way a topic list looks, which is usually the title, but themes and theme components can add an excerpt. Activity focused on topic is, for example, also used for the voting activity /my/activity/votes. It allows a quicker overview of which topics you voted for, and the titles usually work as a good summary.
For activity focused on individual posts, showing only the topic title wouldn’t help much. Whether it’s about the user having posted something or liked something, you need information about the post itself, not only the topic title. That’s why showing some content is needed here.
OK, your reply answers the question as to the “why”.
Topic titles unfortunately often don’t reflect the question, especially when a beginner starts a topic. So content is relevant in that case as well. I do not know how often it happens on this forum but forums that I regularly visit show titles like “Help please”, “My code does not work” or a topic starter tries to squeeze a complete code in the topic title.
It would be possible to display an excerpt of the topic on a user’s profile with a custom theme component, the one downside is that our serialize_topic_excerpts flag is global, so it increases the payload a little for all topic lists… though this is usually fairly minor in practice.
We have a theme component that can already do this globally or for specific categories and tags (Topic List Excerpts), we could potentially add a setting to allow it to only apply to user profile topic lists if that would be useful.
It’s not uncommon here, and we’ll often rewrite a title slightly to make sure it carries more context if it’s too simple/vague. I’d encourage staff and tl3+ members to try and help out with titles where possible. Descriptive titles are helpful for everyone… it helps the author find the right person to answer, helps readers find topics they’re equipped to handle, and provides better indexing for future searches.
It’s also possible to configure AI to check and improve topic titles if a site has a volume of new topics that’s too high relative to the available moderators. The Discourse Workflows plugin can be configured to adjust titles however you’d like.