Why?
The spell does not state that the surfaces must be parallel to each other and contiguous. "These masses must be anchored to two or more solid and diametrically opposed points". It could be two candles.
It's magic.
You fly between any two trees, someone casts Web which grabs the branches (i.e. surfaces) of those trees, you are stuck as if it were two stone walls.
In fact, the spell description says nothing about how strong the surfaces must be, nor how contiguous those surfaces must be.
Any assumption along those lines would be you own spin based on real world physics.
It could be two spider webs and the web spell would not collapse and it would be just as strong as between two rocks.
Why?
Because it is magic and two spider webs at diametrically opposed points would be two solid surfaces. Enough to satisfy the requirements of the spell.
The designer's intent may have been for it to be non-flexible contiguous surfaces, but the spell does not state that.