And they mean a lot to those that do hold them...so why oh why didn't the very first Christian creeds state anything about young-earth 6-day Creationism?
Because everyone believed it, everyone believed the earth was young, gap theorists and day age theorists have only come about in recent history, they are not historical throughout the Church.
Also everyone believed God made the Heaven and Earth in 6 days because that is exactly what the Bible says it does. Now there were exceptions such as "I've forgotten his name

" that believed in creation happening all in one day but he definitely believed in a young earth.
Also everyone could read genealogy so they had no reason to question age of the earth.
The fathers of the Church definitely believed in a young earth and 6 literal days
The Ante-Nicene Fathers including the epistle of Barnabas,
Ó Irenaeus against Heresies Book
Theophilus To Autolycus Book
The Book of Origen against Celsus
And much much more.
..and yet they said nothing about young-earth 6-day Creationism, clearly it was never an essential belief, unlike the authority of scripture
They had lots to say about 6 day creation if you read Church history, however they never needed a creed because everyone believed it.
Why would anyone who didn't have to contend with what they thought was good science entertain anything but exactly what the Bible says?