(this is only a very quick and dirty exposition.)
Humans, like every other organism on this Earth, have been shaped by evolution, which has instilled a set of adaptations into them.
Like most other species, there is a competition for mates, reproduction opportunities and resources within the sexes. So, along the ages, humans have acquired a set of adaptations that "maximize" wealth, status, (whether they still work in the current environment as opposed to the ones in which these adaptations evolved is another question). So, in essence, humans who tended to work more, tended to obtain more resources/wealth thereby becoming more attractive reproductive prospects. You also get more opportunities to help fellow tribe members, improving your status within your community etc. In other words, a highly competitive arms race has emerged.
However, only in recent times, intellectual work has become more common, where the size of the input is not proportional to the quality of the output.
There is another dimension to this. Many humans today work in corporate environments where there is an intense competition for advancing in a rigid hierarchical structure. These advances are strongly correlated with income growth, and power and control over many work-related issues, which is strongly related to status. In these environments, it is often difficult to actually measure the output quality and quantity of each human, partly because there is little incentive to do that, partly because people doing the measurement are not the most technically knowledgeable in the field, but mostly because they are humans themselves and thus have their own interests (namely improving their reproductive prospects themselves, so they need to forge allegiances that stabilize their positions etc). Promoting the most capable worker is sometimes outside their interests. So in this environment, many other cues are used to evaluate the worth of employees, many completely unrelated to their capabilities and work. So, marketing oneself becomes an important aspect in this economy.
One clear way to signal to others that you are very dedicated to the company and an important employee is to spend more time at the office than the others. Busyness is a signal of importance/status and value to the company. Of course, people actually interested in performing high quality intellectual work will know that time spent on a task is only one (minor) factor ...