Why do most RFiD tags use hexadecimal numbers?
They don't.  They store a number which is internally represented as binary and transmitted as binary.  There is no hexadecimal here.

Humans have a hard time grasping numbers with large number of digits.  We therefore often represent large binary numbers in hexadecimal when intended for human understanding.  We have gone so far as to make the tools we interface directly to, like assemblers and compilers, interpret hexadecimal numbers for us.  However, none of this has anything to do with the representation inside the RFID chip, which is purely binary.

The following numbers are all equal:


  15F4B3D2 hex
  368358354 decimal
  00010101111101001011001111010010 binary

What do you think the chance of human error is in manipulating the last one compared to the first one?