It's not because of a domestic economy. It's because a freight/boat is much more economical than a train and a train doesn't make any sense.
It's only here ( now ), because it's heavy subsidized by China and an excuse for modern colonialism.
PS. A lot of ( sane / healthy) infrastructure works are happening in Africa with Europe / US though. But if it's not feasible, it ain't going to happen. That's where the Chinese come in to burry them in debt.
I question how stable the relationship is when that stability rests on the assumption China fails to meet its economic goals.
And also quite importantly as China climbs up the value chain it will increasingly seek to outsource to cheaper countries, as it has already started doing in Pakistan, one of the major hubs of the Belt and Road initiative. And given the quite authoritarian system, it will probably do so quite effectively without facing the political backlash that workers in the United States have caused.
The US has no concept for Africa, because unlike China which uses infrastructure projects and investment as a way to offload its export surplus the US is an overwhelmingly domestic economy.
There will never be an American 'Belt and Road' because it just doesn't make much sense for a country that has so little international economic exposure, whenever the US forms alliances it is towards purely political ends. In that sense one could indeed characterise the relationship with China as more stable. No Chinese leader is suddenly going to go isolationist over the next two decades just because they feel like it.