Irregular menstrual cycles are very common and happen for a lot of different reasons. Also, there are different kinds of "regular" periods. Someone could be said to have regular periods even if they happen on shorter or longer cycles than the typical 28-30 days provided that it's a consistent pattern without significant deviation for that person.
You do not have a good understanding of menstruation and gynecology if you think that's always the case. There are so many variations of irregular menstruation that trying to exclude data based on irregularities would be very difficult or get rid of a lot of legitimate irregular data.
Perimenopause can happen in women a lot younger than most people think. I'm in my 30s and dealing with perimenopause symptoms such as hot flashes and irregular periods.
I'm supposed to be tracking my periods to help my Dr decide if that's what's going on, but because if this anti abortion garbage I have to do it manually on paper which I'm terrible about remembering to do(brain fog is another symptom)
So yeah, irregular periods are common for many reasons(endometriosis for example), but the most common one every ovulating woman eventually faces is perimenopause and menopause.