Real quesion

Why most of the IGLs bottom frag?

because they have to focus on calling and on the bigger picture of the game instead of focusing on their own game.
Edit: Also, a lot of IGLs are old CS veterans, because to be a succesful IGL you need some experience. So they're not likely to have the best aim and reaction time (ex: FNS, Hazed, Steel)

Use ur brain... pretty obvious

They have to focus and manage their team with strats, so it clouds their focus on mechanical skills. Also outside of the server they allocate their time differently, spending more time into research of sorts than just grinding aimlabs.

Thanks for the reply!

More focus on the teams overall play + low mechanical skill usually