Homomorphic encryption - Why does addition not imply multiplication?
There are at least two problems;


The $b$-times addition leaks the $b$. A semi-honest observer can see that you add the $a$ by $b$ times. However, in FHE, the $b$ is also encrypted with semantically secure that leaks no information. The only information available to the observer is the circuit.
In FHE, the $b$ is coming (or may come) from another result, which means that $b$ is also encrypted. In additive PHE, you cannot multiply by $b$ without decryption.


You can look at some example of FHE circuits from this answer to see that some of them are not even possible with additive PHE.