Because floating point zero has the same representation as integer zero, 32 zero-bits.
MIPS also has hardware for double precision (64 bit) floating point operations. For this, it uses pairs of single precision registers to hold operands. There are 16 pairs, named $f0, $f2, — $f30. Only the even numbered register is specified in a double precision instruction; the odd numbered register of the pair is included automatically.
Some MIPS processors allow only even-numbered registers ($f0, $f2,...) for single precision instructions. However SPIM allows all 32 registers in single precision instructions. These notes follow that usage.