without having to pay the 5% fee when selling and rebuying the new cards (so 10% fee total taken from players????)

You have no idea what is 5% fee! LOL

This is the artificially created thing by founders/developers and it is the part of the ecosystem.

We create the post about the inner reasons, if you know how to use the Google Translate - you may read it (or enough to you to look at the pictures/screens of this article):

https://splinterlands.ru/komissiya-za-prodazhu-kart-formiruet-prizovoj-fond-turnirov/