I see this is a hot topic within the community at the moment! Thanks for posting - even when someone else has posted similar feedback, it helps to know whether that’s just one person’s perspective / concern / preference or one shared by multiple people.
To respond to your question, it’s just a matter of priorities - 2D art for tooltips is comparatively quick to produce, whereas the 3D art takes longer. The people who would be working on this have been busy with other things, such as creating new models for the Mage, Primalist, and Sentinel classes. It’s the kind of polish which we’ll be able to focus on more during the beta stage of development, which will be from April of this year through to 2020.