Because you can’t. In exchange for a whopping double stats, you can’t customize it very much. Instead, players must build around them if they want to incorporate it on their builds
The only Eternals that are changeable are the ones with four slots (Unreal Hood, Fabled Blade, etc.). These slots can be filled to give the item a Mythic skill, and then you can use Topaz to fill the rest of the freed spaces with affixes.
The higher quality an item has, the better affix rolls you can get. Eternal items have 50% Quality, the highest in the game. You can only use up to Topaz on the Eternal types that I mentioned. Sadly. It would be too OP, otherwise. Haha. However, people will sometimes get perfect rolls due the item’s quality.
I’m talking about normal Eternals. Not the fabled or other 4 or more empty slot eternals. An some Should have specified but I was on my lunch break An didn’t have time to have a longer answer.