Yes, but datapads are easy.
After all, if they were impressive relics, why wouldn't they be given to space marine captains?
Forge Worlds use conditions that make sweat shop barons go "wait, hold on here, workers need some rights", to make more mass produced goods than china could in a year. And they do it in a day.
Every day. On every forge world.
Datapads are just e-readers, and huge rows of menials put them together bit by bit in massive assembly lines.
While you won't find them in the underhive, in the emporiums of the hive itself, there'll be racks of them. After all, they arn't expected to last for centuries.
Now parchment? That's a harder thing to do. After all, while groxen are the most plentiful herd animal in the Imperium, their skin won't do for writing on. So you're going to find most coming from Horseskin.
The most valuable parchment is going to come from the horses used by rough riders. As they gave their lives for the Imperium.
Then it undergoes a lengthy process to turn it into something to be written upon.
A proper piece suitable for the writing of hallowed works (such as the rolls of space marines) would cost far more, and take more specialised artisans than any monochrome dataslate. Even before it's illuminated.