In more advanced countries, the basic elements of government functions are automatically funded UNLESS there is specific action to defund them. Sort of like a household where the mortgage, utility bills, etc. are on auto-pay. You know you're going to have to pay them no matter what--those bills aren't discretionary.
Whether keeping national parks open should be considered an "essential" is an open question, perhaps, but it should be noted that the abovementioned more advanced countries, such as Canada, Britain, Germany, etc. do consider maintaining public land facilities to be just that.
Another major factor is that the US National Park system is severely, chronically underfunded, so the parks don't have the reserves to stay open for even one day after the government says it won't fund them. The problem will only be worse if Trump achieves his stated goal of cutting the National Park Service's budget by 35%.