They don't think they are weak, but just imagine a situation, where you have gone through an entire day at school/work and you take a bus/metro and you find some men eve teasing you.

There are a bit taller than you or maybe more in number or maybe alone, and you do know how to maybe shout, call the police and punch them.

But, then you start thinking.

What if they are Powerful people?

What if they bribe the police ?

What if the police doesn't sympathize with you or what if they put you through a long process on the road to justice and until then, they are free.

What if they have an angry relative who throws acid on your face and runs away?

What if they have friends and because they know your routine they harm you the next time?

So, it is not easy because actions have consequences and unfortunately , the consequences have to be faced by the victims ,not the culprits.
And at that moment , you decide that I won't fight this battle because things will get escalated.

It is going to get bad and mostly, bad for you because people would never talk about the time you got eve teased until you stand up against it.
As if you shut up and keep your head low, you are the good girl who did what was best and ignore the boys!

Also, our parents do tell us to not get involved in such a situation because they could be dangerous and things will never change for women.