areyoubitter - believe me, we DO know all that! We also know that with certain aircraft, it can be easier to fly a track than a heading thanks to the way the FMS is set up. The fact remains, unless there are wildly differing aircraft types involved, drift will have a similar effect on everyone and it keeps everyone on the same page to use headings. It is pretty likely that the next instruction after "fly runway heading" will also be a heading, with luck towards your first en-route fix.
Have a look at some North American SIDs and you'll see lots of extremely simple ones, along the lines of "climb on runway heading to x,000 ft". One of the benefits of such simplicity is that everyone with an IFR flight plan can use the SID, whether in a 1960s Cessna single or a brand new A380.