Continents are not natural features, but human determined. Some of the models don't make logical sense.

If Americas are one continent, then Afro-Eurasia must be continent. And end up with the four continent model with continuous landmasses.

If isthmuses make continent, then get the six continent model with tectonic plates.

If can split into non-geographical regions, then Europe and Asia can be separate continents and get the seven continents in the US model.