It might be sort of complicated for them to implement this as it would need to create an additional database of some variety for each character to track which Transmog items they have. No idea how hard that would be, could be easy.
That said I'm fairly certain there are addons that accomplish this and I do understand some people prefer to avoid addons but, functionality that can be accomplished with addons would likely end up being low on the priority list for the Devs to implement. ( Not trying to be negative, it's a good suggestion and I do agree this should all be base line and not something you need addons for. )
https://github.com/Otari98/Tmog
https://github.com/laytya/EQCompare-Vanilla
I'm not familiar with either addon as I'm not big on transmogs and I believe that compare function is built into PFUI but if you're not into using PFUI that EQCompare addon *should* accomplish that specific feature without requiring all the PF bloat.
<Insomnia> is recruiting. [NA] [N'zoth]
Community oriented Raiding Guild.
We plan to start hosting weeknight raids starting at 9pm EST with Wednesday/Thursday as our main raid days.
https://discord.gg/fEmGDxtphP