It is a bit ironic really because once you fly for an airline you normally wear your "civvies" for ground training and simulator.
However I suppose that from a commercial point of view, it panders to the customer as well as creating revenue for the organisation. Like those adverts they place in the magazines of pictures of pilots in their four bar glory standing next to some jet and pointing into the vague distance.
In reality its probably about image and "spin". The latter was once an important exercise to be taught and learned. These days it seems to be about dressing up for groundschool and flying lessons.