Why do I need to adjust TV antenna on the roof for better video quality?

Picture the radio waves from the TV transmitter as flat horizontal sine waves. You want the antenna to pick up the full width of this wave in the horizontal bars. So you need to point the antenna as directly as possible towards the TV transmitter.

Cell phones send the signal in a vertical wave (apologies for the oversimplification). This doesn't allow you to pick up a clear signal from as far away, but does mean that you can simply hold the antenna vertically and be able to receive a signal from any direction. Cell phones also contain a lot of modern signal processing electronics to handle a weak signal, which weren't available when TV was designed

P.S. You presumably only need to adjust it if you want to get a signal from a different transmitter. Or perhaps if the signal is really weak and the atmosphere slightly changes the direction the strongest signal is coming from.

The main problem comes from the fact that TV signals are highly directional. This means that if your antenna is "slightly off," the strength of the signal being received will drop significantly, causing loss of quality of the signal, and even the signal itself.

With the right combination of mechanical and/or electronic devices, the "pointing" of the antenna can be automated so that it always obtains the best available signal.