
This is a good question.  There are major blind spots in our understanding
of what makes the earth habitable.  For example, why does the earth's
atmosphere have the concentration of oxygen it does?  The naive
answer is "photosynthesis", but this is clearly incomplete.  Photosynthesis
by itself can't make the atmosphere oxygenated, as the oxygen produced
is consumed when the plants decay or are eaten.  What is needed
is photosynthesis plus some mechanism to sequester some fraction of
the resulting reduced material.

On earth, this mechanism is burial in seafloor sediments of organic
matter, mostly from oceanic sources.  However, this burial requires
continental sediments (in the deep ocean, the burial rate is so slow
that most material is consumed before it can be sequestered).

This suggests that a planet without large oceans, or a planet without
continents undergoing weathering, will have a hard time accumulating
an oxygen atmosphere.  In particular, an all-ocean planet may have a
hard time supporting an oxygen atmosphere.

There is also the problem of why the oxygen in the earth's atmosphere
has been relatively stable over geological time, for a period at least
2 orders of magnitude longer than the decay time of atmospheric O2 to
weathering in the absence of replenishment.  No convincing feedback
mechanism has been identified.  Perhaps the reason is the weak
anthropic principle: if during the last 500 MYr or so, the oxygen
level had dropped too low, we wouldn't be here to be wondering about
it.