Fathers (and men, generally) can, of course, be caregivers, but that's only one of a mother's roles. It's far from the only one.
I accept that your "we" is a group of people who don't value motherhood highly. Based on my wife's experiences as a pregnant woman and our experiences being out and about with our children, I don't think that's a common sentiment in the general population.
I'll end with an unsupported assertion and a small mental leap: people are paid for work because it's the only incentive that will make them come to work. Since the 19th century, society has been progressively more successful at incentivizing women to abandon child rearing and home making in favor of commercial enterprise. Perhaps as a result (but certainly correlated), women's sense of well being has been dropping (at least through the latter third of the 20th century). Perhaps this link is causal, and the happiness lost is the reward for child rearing.