Here's why:
* "Excusable" is already a complete word. It describes something that can be forgiven or justified.
* Prefixes usually modify the meaning of the word. Adding a prefix to "excusable" would likely change its meaning to something other than "forgivable."
However, you could use some prefixes to create related concepts:
* Un-excusable: This means something that cannot be forgiven or justified.
* In-excusable: Similar to "un-excusable," this also means something that cannot be forgiven.
* Pre-excusable: This is less common but could mean something that *could* be excused in the future.
Ultimately, the best way to express the concept you want is to use phrases like "highly excusable," "somewhat excusable," or "not excusable at all."