I felt this way at first as well, when I started leveling. I barely hit level 20 leatherworking with the many materials I thought I gathered. But once I started discovering how the game works, how incredibly useful the store collectibles option in the inventory is, I managed to level my profession 90% of the time without having to buy additional mats to get to 400.
It is true, however, that had I only crafted items I could use for myself, it just wouldn’t work to level alongside my character’s level. Then again, it’s also true that I would receive atleast one item I could use.
What I did was make all insignias (with mats I gathered subconsciously while leveling) then combine them with the two parts that make an armourpiece and invent a new recipe, which often gives 1-3 levels at a time, depending.
I am not sure about this, but I think making 15x the same item will not reward you as much experience as inventing a new recipe does.