do you have a source for this? Dred Scott, Plessy v. Ferguson, Roe v. Wade, Bush v. Gore, Citizens United are all split.

Just look at the decision lists on Oyez or Scotusblog? Big decisions are more likely to be split than small ones but SCOTUS hears many many cases that get almost no media attention, most of which are decided unanimously.

Approximately 50% of rulings are unanimous each year now. https://news.bloomberglaw.com/us-law-week/supreme-court-unan...

In the past, it was even more so.