question about hid and height

I've always been informed that ones HPS' should be as close to the plants as possible for a.) maximum lumens and b.) to avoid node stretching.

However, why is it then that these big grow rooms keep their lights hung very high without adjusting the height in accordance with the plant growth? Does it not really matter if one is using 1000 watt HPS?

If it doesn't matter, then wouldn't be easier to keep them at a set height? And especially for maximum light displacement?
 
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