Think again, you set your saddle position in relation to BB i.e. saddle setback. Therefore reach DOES take seat tube angle into account. And if you look at how the reach and stack are mathematically calculated it should ne pretty evident that the seat tube angle is taken into account...
To be able to compare frames with different headset types one has to also take headset stack height into account when calculating stack.
I really want to believe you
BUT!
The world is against you - everyone on the internet defines reach as this
Frame reach is the horizontal length from the BB to the center of the headtube
I really want to believe you
BUT!
The world is against you - everyone on the internet defines reach as this