Why does the iOS simulator require i386 and x86_64 symbols even though I'm on an x86_64 system only?

The iOS simulator can run your app in 32 and 64-bit modes. This allows you to work out a lot of 64-bit issues and make sure it is ready for both armv7 and arm64. To do this, it compiles your app for i386 and x86_64 and requires the libraries for both architectures.

iOS apps need to run on many different architectures:

arm7: Used in the oldest iOS 7-supporting devices
arm7s: As used in iPhone 5 and 5C
arm64: For the 64-bit ARM processor in iPhone 5S
i386: For the 32-bit simulator
x86_64: Used in 64-bit simulator

actually i386 architecture is for iPhone simulator where as x86_64 architecture is for iPad simulators, both are 32 and 64bit compatible.