Pretty much what the title says, why is it that chords like C7 or F7 have a minor seventh instead of a major one? Is there any particular reason to it?
Some think that C7 (for example) belongs to key C. It doesn't. It belongs to (comes from, is diatonic in) key F It's the dominant chord in that key. As such, by 'stacking thirds', the interval between 1 and 7 (C and B♭) is m7.
In actual key C, the 'stacked thirds' give the interval between 1 and 7 (C and B) as M7.