The problem is that they literally can't do that - Docker Inc. took a huge pile of VC money, and containerization is a commodity feature. To succeed they have to build higher-level products and services, which put them in direct competition with the other vendors in the ecosystem.
Great point - it would be nice if they separated out their monetization products (Swarm, Docker Datacenter, etc) from the core Docker engine. There is plenty of money to be made from enterprise customers who will want the security features of Docker Datacenter, or support for Swarm, without bundling your orchestration layer with the container runtime engine.
This type of bundling reminds me of Microsoft bundling IE with Windows. Initially, IE was much worse than Netscape, and this seems like roughly the same thing - monopoly in one market bundling an inferior product to try and achieve a monopoly in another somewhat related market.
If they didn't at least try to have a complete solution, someone else would package docker as one part of a turnkey solution and they would get stuck with all the maintenance costs and none of the consulting fees.
What annoys me about it is that it requires everyone else to lose for Docker Inc. to win. It's rather like if Linus invented Git, then formed a VC-backed company on the back of it, and then tried to figure out how to extract enough money from the ecosystem so that Git, Inc. could be flipped or IPO'ed for a really big pile of cash.
