I don't see any justification to put drag racing in the game (it would be incredibly boring considering there are no turns and it is basically like running a quarter mile time trial). Drag racing might be fun to watch in real life but I don't think any video game could make it entertaining. Hence why there has never been a drag racing game released to my knowledge (and if it did it probably sold less than 500k).
Well there a lot custom rooms in online with players who want to compete against each other in drag races... the GT4 have this option for one track (Las Vegas)... and if you look at youtube you can find so many videos about Drag Races in GT5.
It's the 5th most rated feature requested to PD implement in GT5: http://feedback.gtplanet.net/forums/96709-gt5-feedback
I think the gamers like Drag Races.