Why do I need a lawyer to get married

The question is, what is it your are asking if you need a lawyer to do?

Anyone can get a marriage license and get married, assuming they are otherwise able to marry (of legal age, not still married to anyone else, meet all other legal requirements as spelled out pretty clearly on the forms). I would never claim you need a lawyer for this alone.

But, what ELSE are you seeking to accomplish aside from getting married?

Protecting assets (you posted in "marriage/prenuptuals" - so I'm assuming you may be thinking about this) - here timing may be important, and you may want to think about speaking with a domestic relations lawyer before the marriage happens. You will probably each need your own lawyer.

Get a green card based on the marriage? You might want to get an immigration lawyer; you can't really count on USCIS for advice. Did USCIS tell you that if you enter on a B-1/B-2 visitor visa and then get married within a certain period of time after entry, there is a legal presumption that you committed fraud by not really intending to "visit" briefly as required but rather stay permanently? Didn't think so. Did they tell you what to do if the U.S. spouse's income levels don't meet the legal requirement? Did they tell you how to respond to the question about whether you have ever been "..cited, detained, arrested, charged ..." when you have never been arrested or even charged, but you and a friend were stopped and questioned back when you were much younger in your home country over something the friend did? Will they advise you how the interview will go, or what type of green card you will get and what you will need to do to hold on to it?

There is a bit more to it than just the forms.

Speak with a lawyer specializing in the area where you want to get something accomplished. Good luck in your marriage.