I can't find a reason why invisible and insubstantial creatures do *not* provide cover, but this seems ridiculous.
PHB says: When you make a ranged attack against an enemy and other enemies are in the way, your target has cover.
For example:
-------------
R . . . (p) P
-------------
R = Ranger
(p) = invisible and insubstantial fey panther sneaking up on R
P = not invisible or insubstantial fey panther R is shooting at
P has cover from (p), but as a DM you can't say why, right?
On a related note, does a Fighter who's blinded and immobilized get a Challenge attack on an insubstantial, invisible creature moving away? Yes this really happened!
PHB says: When you make a ranged attack against an enemy and other enemies are in the way, your target has cover.
For example:
-------------
R . . . (p) P
-------------
R = Ranger
(p) = invisible and insubstantial fey panther sneaking up on R
P = not invisible or insubstantial fey panther R is shooting at
P has cover from (p), but as a DM you can't say why, right?
On a related note, does a Fighter who's blinded and immobilized get a Challenge attack on an insubstantial, invisible creature moving away? Yes this really happened!