If plug-in sleeves are not sufficient for fastening in the ground, why do the posts have to be cemented in?

Concreting the posts in the ground is absolutely necessary in order to achieve a permanently stable and secure attachment. We urgently advise against other methods, as they do not achieve permanent stability.
Our attempts to use push-in sleeves have turned out to be useless. If the sleeves were too large, the posts were then too loose and not firmly anchored. If the dimensions of the plug-in sleeves were too narrow, there was the problem that dirt and earth quickly settled between the contact surfaces and it was therefore not possible or only possible with great effort to pull out the posts.