That is one of the great questions of life...
And unfortunately, the "why" will never be explained. All we can do is walk along ALL hopeful wrong paths until we have the one that is not wrong...
I know about these telescopic shafts. There is one downside, they are to the best of my knowledge not availlable in the length that I need... Otherwise I would have ordered one long ago.
What I need is one that has a safe extendable length under load of 24 cm.
I have not yet seen them longer than 22 cm...
They are usually intended for Crawlers, so probably not that suitable for heavy loads.
Which is a real shame, since these splines look like they would slide excellently under load...