Because floating point zero has the same representation as integer zero,
32 zero-bits.
Register $0 works as a floating point zero as well as an integer zero.
(Floating point "negative zero" has the sign bit set, and all other bits zero. It is rarely needed.)
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 you to use all 32 registers in single precision instructions.
These notes follow that usage.
Remember the load delay, where it takes an extra machine cycle after a load instruction before integer data reaches a general purpose register? Do you think there is a load delay for floating point load operations?