As a tent manufacturer, of silnylon lightweight tents, I do recommend them for my tents. I've seen tents come back after a thru hike when a footprint wasn't used. There are a LOT of micro abrasions and wear spots that are very thin on the floor of the tent.

I sell tyvek footprints, but tyvek can be heavy, and bulky. Polycro or plastic sheeting works well too. it packs up smaller and weighs less than tyvek. The 'big' manufactures usually have an even heavier footprint that is a pu coated nylon.

As to why they don't come with the tent - not every one wants one, just like stakes - they can be a very personal choice,