Why are (3-methylbut-2-en-2-yl)benzene and (3-methylbut-1-en-2-yl)benzene not stereoisomeric?
I thought the only time you cannot assign E/Z is when the product is either mono or unsubstituted.


Well, you cannot assign E/Z when it doesn't make any sense.

tl;dr: When on at least one side of double bond two identical groups or atoms (e.g., Hydrogen), there's only one way to arrange substituents near double bond.

E/Z is used to explain how substituents on different sides of double bond located, which excludes 1,1-substituted ethylenes (which is the case with minor product on your figure).

When on one side of double bond is two identical groups, e.g., methyl (which is the case with major product) there's no E/Z isomerism too, because there's only one way to arrange substituents. You can actually think of first case as of the second, minor product has 2 identical atoms at one side of double bond.

Hope I haven't made more confusion than there was.

Also, I would suggest to play with some ball and stick models (They could be at the uni library), I think that building couple of examples and trying to rearrange them to stereoisomeric form will help with understanding stereochemistry in general.