Yet it still gets grouped in as "heterodox."
They don’t have heterodox people (eg marxists, Austrians, or MMT). Nor should they. CS departments won’t hire people who (for example) believe computers work via magic. Dollars and tenure lines are scarce and can’t be wasted on BS. There is a lot more diversity of thought in economics departments than commenters here know, but there are limits, as there are in every field.
This is literally the foundation of macroeconomics AFTER Keynes. This is (one of several) reasons why Keynesianism was rejected.
Every single macroeconomist in every mainstream department in the country (and everyone in a central bank anywhere) would agree with exactly what you said AND say that it is precisely that fact which makes macroeconomics really hard.
This is why, for example, even the bare-bones simplest macroeconomic model you learn in your first year graduate macro class must include some notion of the agents' expectations.
It would be an error if we didn't do that. But fortunately, it's been at least 50 years since we started thinking about exactly that problem.
Econ departments don't have Marxists for the same reason philosophy departments don't have Buddhist scholars: it's just not where they belong. The Buddhists belong in religious studies, and the Marxists belong in the political science department.