Time signatures with 9 in the numerator are most of the times 9/8. Cases of timesignatures 3k/8 are a bit special, because they they the role of a ternary subdivision. In early music a note could be subdivided in two or three smaller notes, and this is basically the same way. A 3k/8 measure will be seen as a k/4 measure with each beat subdivided by three instead of two: 6/8 is a measure of 2 groups of 3 eigths (~ 2/4), 9/8 a measure of 3 such groups (3/4), 12/8 one of 4 such groups (4/4).
So measures of the form 3k/8 are special in the sense that they are regular, regular meaning that the measure is subdivided into equal groups of two or three.
Now, there are two mainly used measure featuring a 7 in the numerator: 7/8, which is irregular and usually split into two groups of 2 and one group of 3, and 7/4, which is usually regular but with 7 equal beats in one measure.
Now the human is not in fact particularly good with internalizing rhythms. You can easily feel two, three or even four beats. 5 beats is still possible, but try to feel 7 beats without counting. The point taken is that the human as a prevalence for short rhythmic groups, which affects how dances and music work. (You can also draw parallels to stress speech.)
And this means that you are quite unlikely to find something like "7 equal beats", the same way you will seldom find "8 equal beats" or "9 equal beats" and so on.
Additionally regularity is something that gives a nice structure. Dances and music with irregularities are harder to conceive and perform. This means there is a huge bias towards regular structures. In fact you still find this today. Most dance music is simply written in 4/4.
So regularity is an important aspect of out musical language. Also you should keep in mind that in the origin is of the tactus is not filling up a measure with n small notes, but it comes from taking a whole and dividing it in parts (which can then again divided). This is something where you need to actually put in effort to not derive something regular.
Lastly you should keep in mind that notated regularity is not necessarily performed regularity. Playing music strictly to a meter is something quite recent, and traditionally much music would not in fact be played in the exact meter given. Traditional European music has many cases of meters with elongated beats. For example eastern european/turkish music as a traditional 4/4 with an elongated last beat, leading to a 2+2+2+3 9/8. Viennese waltz traditionally has a slightly elongated second beat, which makes the 3/4 into something between a 3/4 and a 2+3+2 7/8. Also in traditional music you’ll have cases of changing meter. Here in southern german area for example we have what we call "Zwiefacher", which is a type of dance that switches between 3/4 (waltz) and 2/4 (dreher) (and does not even do so regularly!).
So while it is not necessarily notated you do sometimes get the irregularity in performance.