That is an artificial limitation. I got an allocation as a ::c001/64 and can use up to 15 more addresses, but I strongly suspect that I share that /64 with many others. I continue paying DO, but I believe they just plain don't get IPv6 and DNS want to spend the time learning. It's a shame as they are likely to be affected by the IPv6 shortage much more than the larger/more expensive providers and it would be in their interest to have more of te Internet upgrade.