Q: because the scale goes like this so there it makes sense and you don't need the b.
A: right. minor or major refers to the 3, but Cmaj7 has a maj7 by association and Cmin7 has a min7 (flat 7) by association. The only weird part is when you say C7 it means major third, but flat 7. that's just what it means. (it's shorthand for C dominant 7, and the word dominant doesn't make any sense there but that's just what it's called) it's just a weird quirk of spelling, like spelling quirks in english or other languages