Why are the pins RC3 (SCL) and RC4 (SDA) configured as inputs during I2C initialization in PIC18F?
It is because the PIC I2C bus is a multi master module, so it needs to detect the bus is free before sending data to it. The requirement to configure the pins as inputs are in the datasheet (I'm using 18F4550 as a reference):


  The MSSP module in I2C mode fully implements all
  master and slave functions (including general call
  support) and provides interrupts on Start and Stop bits
  in hardware to determine a free bus (multi-master
  function).
  [...] The user must configure these pins as inputs by setting
  the associated TRIS bits.


(http://ww1.microchip.com/downloads/en/DeviceDoc/39632e.pdf - Page 209)

EDIT:
I just realized you specified master mode. In this case, refer to page 228 of the same datasheet:


  In Master mode, the SCL and SDA lines
  are manipulated by the MSSP hardware if the TRIS bits
  are set.