Why do midlife crisis husbands blame wives?

Its easier than taking responsibility for the results of poor choices on their part. 

Well, obviously it's not their fault. (sarcasm) 

My husband screams, "It's all your fault!!!"  I just laugh (which does not help).

I turn into a sarcastic b***h. Wow, I have so much power.  But if you no longer wish to live with me, that's okay.  Maybe your sister would be willing to be your roommate. 

The husband is feeling old and frustrated.  He hasn't been able to do many of the things he used to do. 

His wife probably contributed to part of his midlife crisis.  Not all, not most.

But it's definitely easier for him to blame her.  

You live in a judgmental society where there is always someone to blame for anything.  As it can't be ME it must be her, or the government, or big business, or.......

Why do they blame wives for what?

Because he feels that he was used to produce babies and look after your wants and needs regardless of his feeling or happiness. Now, at this stage of his life, he is looking back and saying to himself..."I could have done so many other interesting things with my life and instead, I became a meal ticket"   

Do they? Broad generalizations never apply across the board to everyone. They are personal opinions, and usually wrong.

Because, in your particular 

case, you ARE to blame.

small d!ck energy