Why is the artwork important ? Well, you could also buy some simple stock images and icons (or get them designed from innumerable free or low cost options available on the internet), but the problem is, the artwork is part of the overall branding of the product and has a significant role to play overall. For those who monitor the overall branding of products, when companies make a change in their branding or in the icons, it is a big effort.
From time to time, there is a need to refresh the artwork used in the product, it makes the application feel fresh. It is not so easy to perceive, but users get a bit jaded when they see the same artwork, the same icons in the product across different versions. When the artwork is refreshed, it gives regular users of the application the feeling that they are seeing something new, even though reviews may not give too much importance to the change in artwork. Further, when there are changes to the look and feel of the operating system on which the application works, there is a need to make changes to ensure that there is a sync between the operating system and the application. For example, it could be that the new operating system has icons that have a certain amount of transparency, and applications that do not have the same kind of look and feel stand out (and that too in a negative way, not positively).
However, refreshing all the artwork, or even part of the artwork is not an easy task. It cannot be done without an expert - you have visual designers who talk to the product management, who talk to the product team and senior management, who talk to the customers (we actually had meetings with a group of customers to get their feedback on different sets of proposals for new artwork to see which seems to work, and which does not).
The artwork design and creation process has a separate schedule and is normally done outside the base product team and their schedule, so there are some complications that need to be overcome. The project / program manager needs to ensure that the schedule for this delivery has to be done before the dialogs overall delivery is complete, including some time for evaluation and review, and rework. And then the dialogs / screen would need to be shown to outside reviewers to get their overall feedback and impression, and corresponding changes would need to be done.
There is further impact. Since the artwork is changing, all the screenshots of the application in the Help documentation (in the base language and in the other languages in which the application is available) need to be changed, and this can be an intense effort that takes time and a lot of work (including testing that the change has happened in all the languages).