I guess it's just the money that counts. On a 50-100$ TV-card, companies won't include features if they aren't really needed.
99,99% of the people who want to add a TV-card to their PC, have a PC with soundboard (be it onboard or PCI).
Since there's no synchronisation problem when using a seperate soundcard, there's no need for them to add a soundchip to the TV-card.
Most professional videocaptureboards feature a soundchip, to make sure that analog audiocapture is possible at 48Khz, 16 bit stereo. Since driver-issues are very common for those thingies, it's not wise to hassle with seperate soundchips. Those cards cost about 500-1000$, so customers want something for their money.