That wouldn’t work. You can’t really legislate that a particular action must bring about a specific charge. You’d still have to prove the charge. It would just lead to some attackers not getting charged at all, if the only options were attempt murder or nothing.
In any case, the reason attempt murder is used very sparingly is that it’s notoriously a difficult charge to prove. You’re basically having to prove that somebody intentionally tried to do something that they didn’t actually manage to do.
And with the maximum sentence for Section 18, with intent, being life imprisonment anyway, it’s usually the safer option to ensure someone who’s clearly shown serious intent to harm, doesn’t end up walking.