You're totally right if you look just at the financial aspects you mentioned, but we have to keep in mind that when a spouse leaves the family, the remaining spouse is left doing the things that the other spouse was doing, and this has a cost. For example, the surviving lady cannot take care of all the things that used to be done physically, at no cost, by the man, so there is a cost. So 50% might look correct, but there are additional costs when a spouse dies, and that's one of the aspects.
The other point is that the financial situation of each family is so different.