I've had to deal with stuff like that in lots of older code.
The monitor resolution was relatively stagnant for most people for a long time from mid 90s up until 2010 or so. Smaller monitors normally had lower resolution.
So with resolutions of 1024x768, 1280x1024, 1368x768, 1440x900, 1920x1080, etc. so the size of an icon was relatively similar for the large majority.
It would be crazy to develop commercial software with that thinking currently or in the last 10 years. But for DDO (and so much more), that code was already done, and most likely in an old (depreciated) framework that almost nobody uses to develop new projects. For some projects, the better approach might be dumping the UI and creating a new one but it is a lot of work and often has lots of hidden gotchas that need to removed. Without examining the DDO source code, it is hard to estimate the pain level. And I suspect this is not high on their priority list unlike lag issues.