A valve stem seal, which prevents oil from the camshaft case from penetrating into the combustion chamber, is mounted on its valve guide simply by fitting it on with tension. Yet it endures probably millions or tens of millions of reciprocation movements by the valve. How does a simple tension fit, which is relatively easy to undo even without a tool with your finger pinch, endure such abuse? Does the range of motion of the valve spring/retainer obstruct its way out of its lodging?
For me it is a combination of little oil in the valve stem, reducing friction and drag, and the fact the seals sit with little tolerance to pull force (some even have rubber inside) and the protrusion they sit in ain't polished; you can remove them by lifting them sidewise but if you apply an even pull force (as in the case of the valve stem) they won't easily go.