It does not have to. In fact, it makes just as much sense to say it doesn't exist. Because if we have some life force, where is it? In which part of the body does it reside?
all of it
as a bioelectric charge if nothing else
Why can we not detect it? There is no fundamental law of the universe which states "living things must have some mystical life force."
Life has the quality of life
Your thoughts and the way you think is based upon how the different regions of your brain are interconnected. There is no reason to say "There must be some life force inside us all." Although, just to be clear, I'm not saying that it is impossible either. I'm only saying that it is possible to explain life without some unknown factor.
A creature without a soul for want of a better word is a machine
it eats reproduces and breaks wind
it does not play games or write poetry