The cabins of airplanes are pressurized and temperatures is internally controlled. Why are all planes constructed this way and what would happen if theses systems failed?

The air pressure in an aircraft (cabin pressure) is kept higher than the atmospheric pressure at high altitudes to avoid discomfort to passengers due to low outside air pressure. This process is called cabin pressurization. An increase in cabin altitude is actually a decrease in the cabin pressure.

temperature in the aircraft is internally controlled according to the temperature zones throught which the aircraft is flying.

planes are constructed in this way to ensure the comfort of the passengers flying. And if it is not constructed this way, planes won't be able to fly high and long.

there is a very less chance for these systems to fail. Just in case, if the cabin pressure fails we have oxygen masks. If the temperature controller fails, it is either going to be too hot or freezing cold.

I am not so sure if they have an alternate for temperature control.

They are this way because they have to keep oxygen in the cabins, they are at great altitudes so there’s a lack of oxygen and warm temperatures. If these systems failed the oxygen masks would drop and the cabins would drop in temperature, but it wouldn’t occur at a fast rate.