I think most supervisors try to discourage students from writing in this way because there is a delicate art to it that you are unlikely to be able to meet at that career stage. When I read some of the texts from Martin Hairer and Cedric Villani in my field (or even Riemann's famous paper on the zeta function), you can see that they are motivational and informative but still concise at the same time. Full of personality too, especially Villani. That's a hard balance and it requires exceptional understanding of the topic and the reader.

At the same time, we really should be encouraging it more. I have found that in newer machine learning theory papers (strictly theory, not empirical work), there is something closer to a good balance that is expected even of students.

Including a motivating example is key to this, and should be considered mandatory.