Why should I replace the lamp only instead of the entire lamp housing unit?

The lamp is the only component on the lamp housing unit that will go bad.   It is similar in this way to a light bulb in your home that will break or burn out over time and need to be replaced.   Much as it is more cost effective to buy a new light bulb instead of a new table lamp, it is less expensive to replace your TV lamp instead of purchasing the entire housing unit.   Simply replace the lamp, reuse your housing and achieve the same great picture as when your TV was brand new.  

Replacing the lamp in the housing is easy and is environmentally friendly.

There is no need to replace the entire plastic unit - you can re-use it with a new lamp and achieve the same result as replacing the entire unit.