The negative buss is usually very close to the DC breaker panel. DC wiring
is run with two conductors in an external jacket and you don't want to have to strip off a dozen feet of external jacket to make a negative termination a dozen feet away from the DC panel and another positive termination at the DC panel circuit breaker.
FWIW there is usually another "primary" negative buss close to the engine starter and the batteries. That buss grounds the engine and also has a #8 or so gauge wire that feeds the secondary DC breaker panel negative buss. The negative post from the batteries grounds to that block as well.