The hydraulic shocks found on exercise machines aren't very different to the shocks found on cars, bikes and trucks or even inside some washing machines - the ones on display on bikes and exercise machines are better finished and nicer to look at.

Your question is asked everyday by thousands of vehicle owners who also want to know why shocks leak.

I have a theory, but it is just a theory - If a telescopic shock absorber is going to leak they mostly start leaking while relatively young. I think very few leak because the seal is worn out - a very few do and that happens after a long service life.

I think in some cases it could be the quality of the seal or the way it is fitted but my theory is about the ambient temperature, the temperature of the components and the temperature of the oil that fills the shock.
It is possible during manufacture a certain volume (rather than a weight) of oil is placed in each shock and at certain times, such as first thing after the weekend, in the winter perhaps, the oil will be cold so when the finished item meets hot weather the fluid expands and the level could rise enough to overwhelm the seal...