I am yet to see an anti-virus software that's using the native Windows theme. Why do they prefer custom skins over native windows skins? Does it have anything to do with security and/or consistency? Or is it just about the looks?
Some viruses replace AntiVirus software with fake duplicates that look the same way. That takes more effort when the AntiVirus software doesn't simply use the native skin.
The business of AV companies is confidence.
They sell you on the idea of auto-magic security, but have no actual way to show you that it works that way. Because, of course, it doesn't.
So they rely on the a set of design and interaction cues not entirely dissimilar to what a slot-machine maker would do. The overly-designed shell is just part of that pattern. Makes it real-er.
And the winnings of perfect automatic security seem so much closer, when it's all bells and whistles, wouldn't you agree?
There is no technical reason for this. But plenty of engagement, retention, and other reasons, all very commercial in nature. Taking apart a screenshot of a shell like that and putting it back together for a malicious purpose shouldn't scare a script-kiddie. That's part of what they do, don't they? Copy and paste.