If one would write ~pagi instead of paghi (you pay) the writing would be regular, but the pronunciation very irregular. In other words, we have an irregularity in the writing to keep the regularity in the pronunciation. If there is an irregularity in the writing in order to keep the regularity in the pronunciation we can say as well that these verbs are not irregular at all but very regular. Languages are mostly spoken, not written and the important thing is the pronunciation.