So, apparently this is a very very old game mechanic, but is still useful to convert all the useless materia into crafting or gathering materia, but it is so incredible tedious to do.
you have to manually hand in 5 different materias to the goblin, and every time change the amount to 1 and click Ok, 5 times... for every transmute.

Can we have a better way to transmute a large amount of Materia at once?