such 在这里相当于名词,作主语,构成such.that 译为“这是”.such指的是 many people do not value the things they possess until they have lost them.这句话.而so不能充当主语的成分.
例句：Such a disaster as this had never happened to her before.以前她从未遇到过这样的灾难.
The knot was fastened in such a way that it was impossible to undo.这个结打得很牢固,根本解不开.
Such advice as he was given proved almost worthless.他得到的那些建议,被证明毫无价值.
He's not such a fool as he looks.他并非看上去那样蠢.