probably because a large collection would take a very long time to give an accurate valuation. Single items of high value have fairly set values, unless it's a shade/watermark etc variation. Also a valuation featuring stamps would probably only be of interest to stamp collectors, who probably know the value of their own collection.
They have been featured in the past on some of these programmes, but only when on letters or postcards of interest, when the value is in the subject matter.