Not sure why you'd be shocked, it's a solid language for enterprise services like Apple offers, and their other languages - C/C++, Objective-C, Swift - aren't very kind for web services.
Great use case for Go though, especially its concurrency features for web crawlers. I reckon Scala could work too, although it's a lot more complicated / clever.
I would guess because the input sanitizing requirement is harder for the web; having a stackoverflow when running locally requires the attacker to execute locally -- having a use-after-free from port 80 would be a much wider audience
Some Apple services were written in C/C++. One downside is it's very hard to source engineers across the company who can then work on that code, or for those engineers to go work on other teams.
Great use case for Go though, especially its concurrency features for web crawlers. I reckon Scala could work too, although it's a lot more complicated / clever.