That is because the way actual difficulty is calculated is something like this:
<item difficulty> + <material difficulty> / 3
The 3 at the end is different for some craft skills, I think it’s 4 for some but cannot remember exactly.
The reason bowcraft is such a pain is because the highest possible <item difficulty> you can get is 70 for heavy xbow (compared to 90-95 for brestplates and key rings for other skills).
So at around 122 skill, right when you can use blood, you get:
70 + 122/3 = 110 … which is the absolute hardest item you can craft but still only takes 110 skill, which is 12 below your current skill and as such gives crappy gains.
You could try thunder crossbows. They require black pearls to craft I think and should have a base difficulty much higher, like 130? I couldn’t be bothered with the pearls tho.
I pretty much dropped fletching at 123 skill. It takes several 10s of thousand logs to get 0.0-0.5 skill at this point and it’s not worth the time lumberjacking 🙂