Electrolytic caps are just very well sealed - but they're not hermetically sealed. You have a rubber 'bung' at the bottom where the leads come out.
If you think how little electrolyte there actually is in a small electrolytic cap (by the time you've accounted for the volume of foil and separator) it doesn't need to loose much. The capacitor doesn't need to visibly 'spill its guts', slow diffusion of water vapour through the seal itself, the leadthrough joints and seal to the can joint is enough over time, especially when hot.
If you check capacitor datasheets, you will normally find that they quote a longer life for larger can parts (higher volume to seal ratio) than the smaller package ones of the same model, even different aspect ratio can make a difference.
It's also the reason for being careful to avoid stress on the leads, overheating the joints, bending capacitors sideways on the PCB and this can cause premature failure. The datasheets normally specify limits.