This question already has an answer here:
They changed Thanos' motivation for killing half the universe's population in the movies. In the comics, he did it to impress Mistress Death. In the movie, his reasoning was to reduce overpopulation in a universe of finite resources. While halving the population keeps the number in line with the comics, it makes no sense.
Take Earth as an example. Earth has a population of 7.4 billion people. Reducing the population in half brings us to 3.7 billion people. This was the population in 1970. He set us back 50 years. And in reality, it will not take 50 years to recover, as science has progressed since 1970. This means people live longer, healthier lives. People can have children later in their lives. Diseases will have little to no affect as they did 50 years ago. Childbirth is safer etc. This being the case, Thanos would have to halve the population at least once every 50 years.
I doubt it, but is there any other explanation? Perhaps in a tie-in comic?