Because lesbian, trans and generally queer women exist?
Even if you were to only care about cis hetero women, it is silly to think that transphobes will only keep attacking trans women. As lot of cis women have been "transvestigated", have been harassed in bathrooms for "looking trans" and so on.
So even cis women will ultimately suffer if no one speaks up against bigots. The window of what they are allowed to wear, look like, how they are supposed to behave gets smaller and smaller.
The enemy is the same. The same patriarchal reactionary ideology that wants to punish any difference to the imagined norm. Ultimately it is a class war and the rich pricks are winning.
It is liking saying "why should I defend Jews, I am not a Jew.". Sure they might not come for you first but they will come for you eventually. United we are strong. Divided they will get us one by one.
If you are pretend to be feminist but throw your trans sister under the bus, you are a traitor not a feminist.