It's a voluntary design choice since the beginning of Python to avoid the very common mistake of doing:

    while continue = "yes":

instead of:

    while continue == "yes":

Those mistakes introduce bugs that are hard to spot because they don't cause an immediate error, linters can hardly help with them and even a senior can make them while being tired.

Probably not, since expressions can already be statements. But that would allow dangerous code like "if a = 3", which I don't think the Python devs would want to allow.