Why do people find modern work life boring?

Because it really is. It is one of the hypotheses put forward by Desmond Morris in his excellent book, The Naked Ape.

According to the ethologist, humans, that from his point of view are just primates who have started to predate, are still essentially what they have always been over their entire history: hunters.

The activity of hunting occurs inconstantly and intermittently, at undefined times, in sessions that may be lightning-fast or seem eternal, spaced out by mere moments or entire weeks. It is a process without rules, where one must explore/interpret the environment, and also unique, in the sense that each hunting session is something in itself, unexpected and bearer of both negative and positive surprises.

The exploration of the unknown in search of a reward, filled with anticipation, is something that like few others triggers dopamine in our brain and makes us feel alive.

In a way, then, the reason why the work of the modern world is so oppressive is that it is too dissimilar to a hunting session: it is monotonous, dull, almost always predictable and with predefined boundaries. An invalid substitute for the dopamine that man has obtained in most of his history by other means.