Can a stranger enter your home and start living there as a part your family ? No, right ? Howcome Paki's, B'Deshi's, Nepali's enter India and live as Indians ? Is Indian defence at Borders Not Secure enough ?
The answer is our leaders are not strong enough to kick them out, our citizens are not aware enough to notice them and hand them to police, our police is not honest enough to keep them in jail and deport them.
What can a few border security force can do whereas crores of Indians can't.