Transition time (rise time) and propagation delay
I think it's because of the way propagation delay (say \$t_{pLH}\$) is defined. See the figure:



Conventional, it is measured between the mid-points of the transition.

Now if you define real propagation delay (\$t_{p,real}\$) as the time duration between the point at which input started rising and the point at which the the output settles to final value, then one can write:

real propagation delay = time required for input to rise to 50% + propagation delay + time required for output to reach final value from 50% point. 
$$t_{p,real} = t_{pLH}+t_{in,0\to 50\%}+t_{out,50\to 100\%}$$

So that's why half of rise times are added with the conventional propagation delay to calculate the real propagation delay.