"why is the v chord not an Amaj7 but an A7"
Because it has a natural G, not a G#. That makes the seventh of the chord a minor seventh, not a major seventh, so, it can't be a major seventh chord.
Yet it has a C#, not a natural C. That makes the third of the chord a major third, not a minor third, so, it can't be a minor 7th chord either.
It's neither maj7 nor m7. It's the dominant 7th chord in the key of D. Dominant has arguably the single most important harmonic functionality in a key, and it's neither major nor minor.
Any 7th chord without additional identification as major or minor or something else (aug, dim, or weirder stuff) is nearly universally understood to mean a dominant 7th chord. It's normally created from the 5th scale degree in a major key.
That's not the only place dominant 7th chords are found, but other dominant 7th chords are either constructed chromatically (not diatonic to a key), or else modal constructs - for example, the tonic 7th chord in Mixolydian mode is this type of 7th chord (major 3rd, minor 7th). It's constructed the same way as a dominant 7th chord but in that mode it doesn't have the harmonic function of the dominant. This is beyond the scope of your question though.
Short answer: A7 is neither a major-7 nor minor-7 chord. It's the chord built from the 5th scale degree of the key of D major, adding the diatonic notes in that key a 3rd, a 5th and a 7th above that root (A).