The problem is that "whatever they get" is quite often far, far more than they deserve. Who is to say your (hypothetically) divorcing wife (I'm assuming you are male) won't find a judge who will grant her 70% or 80% What if you had that money going in? What if you had millions and married someone who divorced you a year later and managed to get half your money. What if she married you just for your money and it turned out she had cheated on you. Would you still think the spouse deserves whatever she gets? There are cases where she can get quite a lot.
I don't see how that necessarily follows. I think it all depends on what marriage means to you
I think there are a lot of people out there who would like to believe in the fairy tale but are afraid. I don't mean to denigrate it by calling it a "fairy tale" because I think that for some people it can come true. But the fact remains that for many it does not.
Marriage is taking a big chance. For every couple that KNOWS they are meant to be together, there are 3 or 4 that are not sure. Sometimes even the ones who are certain learn that they were wrong. But if they are right, they don't want to wait until they are too old to have children to find that out so they are taking a chance.
There are quite often seriously unequal partners, financially speaking, going into these marriages. They are smart enough to recognize they are taking a chance and so they write out a contract so that some automatic state law can't strip one of them of a lot of money.
The fact is, if there were no need for prenuptial agreements, they would not exist. If there were not a whole lot of rich people who got burned badly by taking that chance, no lawyer would ever have dreamed up the idea.
I would be surprised if prenups have not actually prevented a number of bad marriages from occuring before the fact.