While true, it doesn't have to be.

US Constitution Article III Section 2 Clause 2 which says the supreme court's appellate jurisdiction is subject to "Exceptions, and under such Regulations as the Congress shall make." So Congress could, for example, require the supreme court to hear every case appealed to them instead of letting the justices pick their cases. Or it could require judges to recuse themselves if there is a conflict of interest*. Or it could forbid the court hearing cases where an en banc appeals court was unanimous.

* which could have more teeth than the self-imposed ethics rules for conflict of interest that the court has now.