You can argue that if both parents pay 50% of the cost of childcare, it makes it profitable but that's not the way most couples look at it.
Yes absolutely, that's the point and the problem. Lots of couples do think of it that way, although there are also others who think it's the responsibility of the mother even if she earns more, and that is exactly the attitude we shouldn't be taking.
If childcare cost outweighs one parent's earnings it does not make sense, to me, for that person to work, all things being equal. It is usually (statistically) the woman who stays home in this scenario. I am happy to clear this up for you smile.
Again though, this doesn't mean the father isn't also receiving the benefit. Both parents profit from the lower earner, whoever that is, being able to work without childcare costs/with reduced childcare costs. It isn't just the mum being done a favour!
There are also lots of couples where the lower earner isn't in profit or is even making a loss in the short term from working, but in the long term they are better off. People may also factor in the ability to get back into work later, career progression, pensions. Lots of relevant factors. There are some couples where it will make sense to work even at a short term loss, others where it might not even if there is significant profit.