The fluid/oil tends to leak out some, leading to air pockets and inconsistent damping. Also some dirt and grime tends to get into them and contaminate the fluid.
The vehicles are so light that is difficult to make a good enough sealing system without generating too much friction. Thus the good stuff leaks out and the bad stuff gets in. Better seals would just bind everything up, the cars are just too light, and we demand too much performance. Devices do not scale evenly, it’s not possible/practical to just scale down real car stuff.