Why is the present participle always regular?

Most rules of the English language have some irregularity [citation needed].

Forming the present participle isn’t one of them: take the infinitive, and add the suffix -ing (in speech. In writing, it’s a bit more complicated due to the irregularities of English orthography in general – but those irregularities are themselves regular, if that makes sense).

Why is this? Even "to be", the most irregular verb in English (with a whopping eight distinct forms, almost every one irregular), has a regular present participle.