You are paying for that ISP-supplied router, either explicitly as an item in your bill or implicitly factored into the overall cost. People will either go with

You are paying for that ISP-supplied router, either explicitly as an item in your bill or implicitly factored into the overall cost. People will either go with the cheapest option if there are any options at all or the ISP will simply save costs to do the minimum necessary to provide internet access. And of course those routers are crap and only do the bare minimum. For example they often have comically bad firewalls/NATs that can overflow their state tables when you do any P2P, run your own DNS resolver or run a home server through dynamic DNS. And then there's buffer bloat. A custom router with OpenWRT and enough RAM will cope with those use-cases and will offer all the nice linux traffic shaping plugins like fq_codel and cake.