That is due to the fact that mushrooms are meant to stay in a forest, in a rainy shady spot instead of trying to think and speak like an actual person would.
*grabs voice modulator and megaphone, pokes out of the bushes* "WE..." *coughs a little, needs more umph, space marines are more booming* "WE ARE THE ADEPTUS ASTARTES AND IT IS CLEAR THAT YOU DISGUSTING GREENSKINS ARE INCAPABLE OF TRULY FIGHTING WORTH ANYTHING! YOU ARE PATHETIC! HARDLY WORTH FIGHTING AT THIS POINT! IF YOU DISAGREE THEN I IMPLORE YOU TO TRY YOUR METTLE BY CHARGING THE CLOSEST ASTARTES BASE LOCATED ROUGHLY 5 KILOMETERS NORTH OF HERE!" "OH, AND YOUR FATHER SMELLED OF ELDERBERRIES!" *jumps back into bush*
"Kicking back..munchin on bolter rounds"....OHHHH DIS IZ GONNA BE GOOODDD....FIUGHTING ON DY...FORMUNS....nom nomn nom's