In the context of the site that's publishing this, which is something almost everyone in those comments misses, the why is already answered - you want to maximize your positive social impact on this world, and (maybe through reading other articles on the site) you've become convinced that a good way to do it is by leveraging professional specialization. Then, after picking a career path (advice on which is elsewhere on the site), you're left with the question of... being successful in that career.

So mega capitalists can maximize their profits.

To invest in your own future? Being successful leads to promotions in meritocracies, which can bring to to retirement faster, offer more interesting challenges, etc.

If you don't care, look for a union or government job where seniority reigns king and do the minimum every day for 45 years.

Clearly, enough people have a "why" that they are more focused on the "how". If you can't figure out a "why" for yourself, then you probably shouldn't bother. No article written by a stranger is going to give you the "why".

If you broaden your definition of job to "anything you work on for a long period of time", I think the answer becomes obvious: a job is a large part of your life. Not wanting to do well at it would be nihilistic.

Read the rest of the guide :) https://80000hours.org/career-guide/