I noticed that modes are all based on major scales. How come there are no modes for minor scales?
It's not true that there are no modes of minor scales. Of course there are modes of the harmonic and melodic minor scales. And especially the latter are used extensively in jazz.
Take a look at this answer for the modes of the harmonic and melodic minor scales.
Simply put, a mode uses the exact same notes as its parent scale, except it uses a different root, or tonic.
There is a false premise in your question - there are modes of any scale.
The modes of minor scales use the notes from those minor scales.
With different sets of notes for harmonic and melodic minors, there will be different modes that emanate from them. Note - I do not include the natural minor as it's already a mode of the relative major.
So - there are seven different modes which have the harmonic minor as their parent, and seven others that have the melodic minor as their parent. All with strange names, sort of connected with the modes of the major scale.
A Mode name describes in a compact, unambiguous form what notes are in a scale.
It's unambiguous because it's based on the notes of a major scale and there's only one set of intervals that describes a major scale.
So if you're told to play C Dorian you know what notes to play—you'd take the C Major scale and flatten the third and seventh degrees.
However—there are several different minor scales. Off the top of my head there's melodic minor (which is different ascending and descending); there's harmonic minor; there's natural minor; there's more too.
If your starting scale was already a minor and you wanted to construct the "Dorian mode" for it, what what you do with the third and seventh? Tricky eh? And the scale you ended up with would change depending on what minor you started with.
So to properly name the "minor scale mode" you constructed you'd need to name the minor scale it was derived from too. So suddenly the expression has gotten a whole lot more verbose.
There's nothing at all stopping you taking an existing minor scale and picking a note other than the first degree to be your tonal centre, but the derived scales don't automatically have a compact, formal, unambiguous name like they would if your starting scale were Major.