>>279595097
Because it's retarded in context of the story. Alchemy is shown as being a mathematically sound science that literally anyone can do given enough knowledge about what they want to transform. Him "giving it up" is equivalent to giving himself temporary amnesia. It's like forgetting how to ride a bike, he can eventually relearn it without issue.
There's also the problem that it came out of nowhere after being sidelined for god knows how many episodes/chapters because helping his dad kill god was more important than his and his brother's journey and the catalyst for the entire fucking story. Arakawa remembered at the last minute that she needs to tie that loose end up and she did it in the most sloppy fucking way possible.