Why don’t I need a satellite dish for my satellite radio? Why do I need one for satellite TV?

It is an engineering trade-off. Using a directional antenna, like a dish, allows you to transmit or receive more information with the same amount of broadcast power.

The amount of power is set by the size of the solar panels on the satellite.

The satellite TV people could have sold receivers with omnidirectional antennas, and then you wouldn't have to point the antenna at the satellite. But then, they could not expect you to receive as many bits of information from a satellite broadcasting with the same amount of power. They could not split their allocated spectrum into as many channels and would have to offer fewer channels of programming.

Conversely the satellite radio people could have sold receivers with a dish, and then they could send you more programming with the same amount of infrastructure. But you would have to keep the dish pointed at the satellite, which would be challenging in a moving car.

Rather than having a literal dish swiveling around on the roof of the car, you could achieve the same result with a phased array of little antenna patches plastered all over the car. The whole roof and/or hood of the car would become the “dish.” Then, all other things being equal, the radio satellite could send you 60,000 channels instead of 200. But the satellite radio companies must have decided this would be too expensive and difficult to install for many consumers to want.