That's a tough one, and there are two distinct answers. The facile answer is that it's been too controversial to ever ram through Congress in some form. Almost no other issue sparks this amount of animosity on both sides. The other answer is that we have a very strange system of government whereby Congress can only enact certain types of laws, mostly to do with finance and the military. Many of the classic criminal laws here (think murder) are state laws. When the feds want something done they pass a law that says something like, "States must do X or else we won't give them any money." Congress passing a law that says "abortions are legal" probably isn't possible. But on the other hand it should really need to be possible, because historically rights have only ever been expanded in this country. They haven't really ever been taken away (except in some really rare and transient cases--during WWII for example), especially in cases where the public so overwhelmingly is on the side of the right that's being debated. So for the average American abortion has been settled for 50 years. The republicans used it as a wedge issue to get votes in the 80s, and since then it's taken on a life of its own (no pun intended) and some of them believe their own bullshit at this point. Public opinion is so firmly against the Court on this one, that I fear it's going to get out of hand one way or another.

b_b's answer lacks the animating mechanics of the creaky old system. As every American schoolchild is taught (and some even learn):

- The executive branch (president) is in control of the resources necessary to enforce law

- The legislative branch (house and senate) is in control of making law

- The judicial branch (supreme court) is in control of determining what laws are just

Does that look like a game of hot potato? 'cuz it is.

Roe v. Wade was a case decided by the Supreme Court in which the court ruled that you could not have a law that prevented abortion, based on their reading of the constitution. From a legal standpoint, a law that says "you are allowed to have an abortion" is like a law that says "you are allowed to chew gum" - it's a superfluous law, and those get struck down, too.

There are plenty of states that have passed resolutions and laws preventing the restriction of abortion rights that kick in if Roe falls. That's legislators setting traps for other legislators in the interest of virtue signaling, but what is the US legal system if not virtue signaling. But if the Supreme Court strikes down Roe, it means that they have determined nobody should have been having legal abortions lo these fifty years.

Then the battle becomes about legalizing abortion and while most Americans support women having the right to abortion, nobody wants to say that. Nobody wants to be the "pro-abortion" candidate. Nobody wants hundreds of millions of dollars of dark money coming in from all across the country to plaster photos of dead fetuses on buses under your name.

It is the ugliest, most divisive culture war issue we have in the United States and the whole "why don't you just pass a law" canard has been used by the Right since about 1974. They know the answer - they've always known the answer. Because people would rather assume this right and nobody wants to run as a babykilller. It'll be the only thing you get to discuss. I mean, we almost passed an amendment that would have entitled women to the same rights as men except, you guessed it, abortion.

Fundamentally, Roe V. Wade has been a stick dipped in a dog turd that the Republicans have used to joust at the Democrats for fifty years. It's been their "nuclear option" - the thing that they can always threaten with because oh holy shit nobody wants to see what it looks like if we actually do have to legislate this nightmare. But since the Republicans have absolutely lost the script, they've chosen to fight the culture war as if they can win.

The abortion rate in the United States is lower now than it was in 1973, and fully a third of those abortions are chemical, which none of this nonsense can touch. It's not a practical thing in any way, shape or form, it's a cultural rallying cry for "men run things no matter what the laws say." But there have been three generations of Republican operatives raised on the notion that this is what they want more than anything in the whole wide world with zero real strategy about what happens next.

So now we get to see who can energize their base more, and how low everyone will go to get what they want. The problem is the Republicans will always go lower, and because of our creaky Mennonite democracy, they have structural advantages.