How could you possibly write such a system? How do you verify that first test in the suite is passed, something has changed in the system to make that test no longer pass? And if you can't, what good is your test? You can't avoid making assumptions; you can just add complexity by making meta-assumptions until you run out of time/money. Sometimes this is cost effective; but often it's not.

If an assertion fails then you don't have a sane environment for your code.

This is frequently true for production systems. It's often your job to deal with an imperfect world, and a dynamic and often flawed testing or production environment. Sometimes, this means accepting sub-optimal results as a consequence of a sub-optimal testing budget, on a probablistic basis designed to maximimize profits for the industry, given departmental budget contrainst.

We don't always live or work in a perfect world. Quite the opposite, most of the time. We have to do what we can within the time and budget constraints we are offered; that's all we can legally do.