Judging by the prices in the various countries I've lived in, in Europe, mobile data prices are a pretty good indication of a cartel.
In my experience Germany is one of the worst (by comparison to what you quoted, I use to get unlimited 4G in the UK for £10/month some years ago) though my own country, Portugal, is even worse.
I bet there were "radio spectrum" or "mobile operator license" auctions won by a handful well connected large companies and there's nothing in the law forcing them to open their networks...