Why shouldn't the Supreme Court totally invalidate the election given it's circumstances and require a do-over?

That would be contrary to their constitutional impartiality and cause a constitutional crisis and major civil unrest.

They don't have the authority

What circumstances are you talking about?  Thus far 18 court cases have been brought up by the Trumpets to invalidate just some of the States voting.  None...none...has been actually accepted by any judge, Republican or Democrat appointee.  In fact some of the judges, e.g., Nevada, are PO'd that the Republican case is so frivolous.

The only persons claiming fraud are Trump and his Trumpets.  And none of those claims has one shred of evidence to support those claims.

In a democracy there are winners and there are losers.  Trump lost, if you love our democracy, move on and accept that.

The circumstances like people being given the opportunity to vote?
The circumstances in which most of them DID vote?
The circumstances in which international observers reported seeing no improper conduct?
The circumstances in which Biden won and Trump came second?
Would these normally be circumstances that would justify  a re-run?
Would these be circumstances in which the supreme court could invalidate the legitimate democratic result?
I think not.

Because the Supreme Court doesn't dole out rulings based on unsubstantiated rumors on social media. 

There is no procedure for a do over.