They are trained to give other instructions first before the takeoff clearance due to the possibility of the following scenario:
Initial clearance was to maintain 6000 feet. You taxi into position and hold. There is an aircraft that is crossing the departure corridor for your runway at 5000 feet:
<ATC> "Airline### runway 27 you're cleared for takeoff, maintain runway heading and 5000 feet for crossing traffic at 6000."
<Airline###> upon heading the phrase "cleared for takeoff" immediately keys mike and says "Cleared for takeoff, airline###"
Now take the same setup:
<ATC>"Airline###, fly runway heading, maintain 5000 for crossing traffic at 6000, runway 27, you're cleared for takeoff"

In the first example, the pilot heard cleared for takeoff and stepped on the controller's additional instructions with his readback. In the second, there is almost no chance of that happening, as if he does stop on the controller with the readback, he'll just be sitting on the runway, expecting to climb to 5000.