The amount of data is growing superlinearly while their resources (probably) aren't, so a dropping price for storage is currently allowing them to keep up (if just barely so).
Eg a 10% decrease a year, means that storing forever costs approximately ten times the first year's storage cost. https://en.wikipedia.org/wiki/Geometric_series#Sum has the math.
Of course, you are putting an exponential decrease into the present value calculation.
To give a truly non-exponential example: summing up 1/n^2 also only gives you a finite value.