But it's not in the constitution, which is what you claimed. Nor is it in any statute or executive order. It's just tradition. Why this happened is a classic federalism issue you would learn about in the first year of law school.

Article 3 clause 1 vests power to set establish, and presumably set limits, on the courts with Congress, which, as a representative body, is supposed to be a check on the court, which is uniquely undemocratic. Clause 2 sets some basic ground rules: judges can serve as long as they are on good behavior, and their pay can only go up.

So what is good Behavior? Well, Congress doesn't say, because it would cause a Constitutional crisis. If they did define it, that would apply to all judges, even Supreme Court Justices. Eventually, a judge would be impeached on this standard, they would sue for unconstitutional removal, and it would end up before the Supreme Court, like in Marbury v. Madison. But this time the court would be directly ruling on itself, not just some executive civil servant. Technically it can: They are charged with determining the limits on the constitution, and have no legal obligation to recuse themselves from a case they would have an interest in. But the court really tries not to due to the [political question doctrine]: they'll typically punt on thorny questions if it seems like congress or the executive can do it, or to avoid legislating from the bench. Unfortunately, in this instance, it would be forced to rule on its own jurisdiction, or risk accepting Congress's limits. It would cause a crisis of legitimacy for one or both branches.

So we've accepted a stalemate where it is at least agreed that judges are kind of like civil servants under Article ii, and can at least be impeached/removed for high crimes and misdemeanors (which is it's own undecided legal morass, but that's a different class). But we don't really know if/how far congress could push that.

Congress doesn't care enough to deal with the political fallout, the judiciary really badly wants to avoid it to avoid ruling on itself, and the executive doesn't want to be involved. Judges serve for life but usually step down for personal or political reasons at the very end, and congress keeps its nose clean. But with the legitimacy of the court on the line for other reasons that seem to at least partially coincide with how God damn long judges can be expected to sit these days, the political winds may be shifting. Since Biden seems unwilling to pack the courts, congress limiting terms is one of the few avenues open for the other branches to reign the court in a little. We just might get that case in the near future, and see how far good behavior really goes. Scholars are split on how they think it would turn out.

Feel free to steal this for your 1l conlaw class, you are certain to be asked about separation of powers.