Steve,

Two answers here:

1)
DDS chips are a fairly recent development. Historically we've been building phase/inc oscillators with discrete logic and micro's for decades. So prior to the advent of single chip DDS chips it was the only way to go.
2)
Presently DDS chips with 14 bit's of resolution are still "pricey" analog product list however their high clock rates coupled with their resolution make them well suited to RF transmission purposes.

In the audio domain the ability to index into arbitrary waveform types with higher bit depths and with user specified configuration (5 series modulators of a single carrier oscillator is not uncommon) is high prized and not yet incorporated into DDS chips.

regards,
p