That is to prevent bugs, for example,
Should &mut map[el] make a new element if el is not in the hashmap?
How about map[el] = value;?
If your answers differed, then Rust can't implement IndexMut for HashMap because there is no way to distinguish between the two cases. Inserting on a mutable borrow is a footgun, because it differs from simply doing map.get_mut(el).unwrap(), this difference would make it prone to bugs.