Because this particular illness isn’t casually transmissible until symptoms start to appear.
It takes a day or two after infection before you start vomiting and bleeding out of your pores. After that it's all down hill.
“Wow. How do they know this?”
Fascinatingly enough, back in late 80’s or early 90’s at a facility housing lab monkeys in Reston, Virginia a strain of Ebola ravaged the lab monkey population - 100% fatality rate.
Turns out that particular strain had mutated and gone airborne. Imagine something as easily transmitted as the common cold, except it can cause various types of hemorrhaging and multiple organ failure.
Thankfully, in that particular case in Virginia, in mutating sufficiently to become airborne the virus also became asymptomatic in humans (not monkeys though, where it had a 100% fatality rate).
Several of the lab workers actually tested positive for the disease, but showed no symptoms. I recall reading that workers in the Philippines involved in the primate industry were tested back in the 90’s for signs of exposure to the Reston strain of Ebola, and something like 20% came back positive.
The events in Virginia inspired a fictional book, The Hot Zone if I recall the name, and that book went on to spawn a movie staring Dustin Hoffman.
The bug has already gone airborne once, it’s just a matter of time and opportunity before it does again or before the extant Ebola Reston strains out in the wild cease being asymptomatic to humans. Even if it doesn’t go airborne, in the age of air travel a blood-borne-pathogen can conveniently hitch a ride on travelers in Africa and end up in San Francisco bath houses in short order.
Hell, hemorrhagic fevers frequently raged through Europe during the last couple of thousand years. That’s the reason that about 1 in 7 people Ireland and the U.K. are resistant to some strains of HIV, and about 2% of Europe shows total immunity to those strains. Selective pressures caused by the repeated waves of disease encouraged the proliferation of persons with a novel mutation influencing T-Cell function.
Whenever you hear about an outbreak of a deadly disease, and an infected person showing up via air travel just a single border away, it would be advisable to treat every soothing reassurance from the government with extreme skepticism until such a time that it really is apparent that the disease is contained. Expect government officials to tell you bald-faced lies just to prevent a panic. Hell, expect them to tell you reassuring fairy tales ESPECIALLY when they don’t have the first clue what the hell is going on.
The truth isn’t going to be available, regardless of whether the disease is contained or not, for months most likely.
They don't. It's the standard lie they tell so there won't be panic.