How can a site like Google have constant, intense web traffic and maintain near 100% uptime, but then when Star Wars tickets go online none of the theaters can handle the traffic even though they knew they’d be busy?

Because they have money and lots of it! They have so many servers to help take care of the traffic it is not even funny!

But…

When Google first released Google Analytics to the public for free (which used to cost a few hundred dollars per month from where they bought it from) Google’s own servers crashed after a couple of days and they had to do have sign-ups for Google Analytics by invite-only until they could get their server resources up again.