I agree that, there is nothing about this kind of habit, in gita."sati" is not mentioned in any of our religious books. but, during those times, wives did not want to live, without their husbands.they willingly sacrificed themselves. But, some people made this habit as a custom, during the middle period. Many Hindu religious leaders, strived hard to stop this habit. The existence of this habit, may also be because of the mu-gals, who has drained our country from it's prosperity and culture.
There was no protection for Hindu women and they have been kidnapped
and captured, forcibly by the mu gals.the men, their husbands ,who tried to stop them were also attacked and killed. We cannot count the no. of temples, destroyed, during the mu gal period. Many cows, that were worshiped as gods, by Hindu people were also captured and killed.There are stories about the people, who gave their life to protect their cows,females and temples.To protect themselves, after their husband's death, they may sacrificed their lives, and this habit(sati) may have spread in other places also.