Why are so many people content with just earning a salary and working 9-6 their entire adult life?

I could think of 5 reasons.

1. Happy and contented : I know of many people who are very happy just being, and have no special drive or goals to achieve. They live life one day at a time mostly, and are at peace with themselves. This definitely works out very well for their families, as they give adequate time to them, and everyone generally seems happy. To be honest, at times I envy such people!
2. Have realised the truth : Some people are of a philosophical bent of mind, maybe even spiritual, and realise that no matter what an individual tries to achieve here on Earth, on a cosmic scale it means literally nothing. There is no permanence attached to anything. So why even bother. A lot of such people may indulge in day-to-day religious rituals, or philosophical thinking, or travelling.
3. Tried and gave up : On certain commonly accepted parameters of social upwardly-mobile behaviour, a lot of people do try during the school and college days. But if things do not materially work out, many just resign and give up all chase. They choose a lower normal (a very subjective term) for themselves, and are either very sad doing so, or as mentioned in point 1 above.
4. Personal, family or health issues : There may be severely restricting personal health issues that can force a person to slow down. Similarly, at least as I have seen in India, senior family members may require a lot of attention (as sending them off to an old-age home is still not the norm) and the younger ones may be required to make certain compromises (especially if it's not a moneyed family).

In general, in today's materially comforting world, where a lot of enjoyment can be had at little expense, a lot of people may simply prefer chasing daily happiness in consumption and life, rather than corporate or business goals that require huge commitments of energy, time and personal presence.

I feel that due to the relative nature of truth, it is wrong in the first place to judge others on this. A person happy with his/her life and routine with no major drive may consider the driven totally mad!

Source: http://sandeepmanudhane.org/Topicwise/default.aspx