And, in many cases, women who successfully breastfeed for the first XX months eventually can't keep up with what the kiddo needs, so, at some point, you start supplementing with formula. And, in many other cases, the well (so to speak) could run dry, so you need to completely switch to formula. Most kids will eat mostly milk/formula until about 1 year old and then you transition to most nutrition coming from actual food.
Bottom line - being committed to breastfeeding kids from birth to the kid being 1-year old is a crap ton of work and requires a ton of time/dedication, etc.