There are other factors. For one, Lightning is significantly thinner, so more suitable for thinner devices. For another, switching to USB-C would force iPhone users (who mostly already have Lightning cables) to buy new cables. I don't know how large a role those factors play, compared to what you said - the other phone makers don't seem to have a problem with the thickness, and Apple certainly isn't known for being hesitant to make its users buy new cables - but they do exist.