Because Apple doesn't yet sell an Apple branded wireless headphone. ;-)
This is exactly my argument. Since you have to have a dongle for anything except power out of the box (how many people previously had any USB-C devices?), then why not a dongle for audio? Why dedicate limited and precious space for an audio port? In comparison, why not put an audio jack on the Watch too?
A second USB-C port would be far more useful than an antiquated audio jack. And to follow the same apologists arguments given for everything else, 'just buy a wireless headset if you don't already have one, no need for a dedicated audio jack, problem solved'. Your observation is a specialized use case, just like all the other anecdotal arguments being offered in this thread. For instance, here's mine -- I've never plugged a headphone into my MacBook, so nobody else should need to do that.
The bottom line is there's no good reason not to include two USB-C ports. The advantages outweigh the disadvantages -- offers a redundant port in case something happens to one, the odds of which are increased now that MagSafe has been eliminated, and the added strain from required dongles. Prevents excessive wear on a single port, resulting from connecting and disconnecting a charging cable twice a day, especially for those who do use it for peripherals, or require more frequent charging. Allows someone to plug in more than one device without buying specialized dongles, and hubs, allowing simultaneous power connection if necessary. Allows connection from either side of the MacBook (a selling point on some previous MacBooks). Does not add significantly to the weight or cost, nor compromise the design.
But let's get back to your original audio port observation -- Apple is essentially using technology that dates back to 1878! So why is that? Why hasn't Apple revolutionized something as antiquated as the audio jack? Wouldn't a connector half the height of 1/8" audio plug be far more useful and space saving in Apple's quest for the ultimate thinness? Something like the size of the lightning connector for instance? To the extent there's ANY justification to keep a wired audio connection on a mobile device, especially one that relies almost exclusively on wireless connections, why not redesign it for all of your products? To that end, why offer an audio port on an iPhone or iPad either? Why not just use the one lightning port for everything? And if you have to plug in anything else on any product, you get an adapter? That seems to be the logic in play here.