I don't know what fucking magical DVDs you have.
Because they're prerendered they're limited to 4 colors.
Because they can be turned on and off they're not hard subs.
Because they have different tracks (laguagues, commentaries, etc.) they're not hard subs.
But it is near impossible to edit DVD subs because they're not soft subs.
Now why would everybody chose this separate low-bitrate video stream that has a high amount of overhead over your pretty hard or soft sub? Because that's what DVDs play. I know that if I use prerendered subs on my production that it's gonig to play in 99% of DVD players and if it doesn't work for you that it's your fault.