That makes it sound like covenants that run with the land are something you could simply implement via contract law, binding the first purchaser to them via contract and requiring that they only sell to people who sign a contract agreeing to being bound and the propagating the covenant.
In fact, covenants that run with the land are stronger than that. The contract approach might work as long as the land was only conveyed through contractual arrangements between the conveyor and the conveyee. But there are other ways land can be conveyed.
My land could be seized by the state because I stopped paying taxes and then sold at auction, for instance. Or I might die intestate and with no close relatives, and the land ends up with some distant relative I've never met or had contact with. Or I go bankrupt and the bankruptcy trustee sells my land to pay my creditors. These and many other ways could result in my land ending up owned by someone else with no contract between me and them, and hence no way for me to impose the restrictions on them.
A covenant that runs with the land would still work after all of those. I've not looked into the history of it, but I'd guess that the need to have a way for arrangements to stick no matter how the land is conveyed is why covenants that run with the land were developed.