Not all Buddhists believe in rebirth.
In Japan, many people who call themselves Buddhists have given up that aspect of their spiritual heritage.
I practice at a monastery where we never speak of rebirth and certain teachers have expressed great scepticism about it.
I personally don't believe in it.
We do still have a notion of 'karma,' but we define it simply as the momentum of tendency ... the way in which all our patterns of perception, judgment, feeling, our sense of 'self' (which is entirely patterned), and a host of other things, present themselves.
The mindfulness meditation which is the fundamental Buddhist practice (in, I think, pretty much all traditions) helps to break up these patterns and allow one to step into the present moment.
Those relatively pattern-free moments are intensely rich ... well worth the effort of doing the practice, even if one has relinquished the notion that the practice will give the biggest of big payoffs -- an improvement of one's 'karma' (by the classical definition) and of one's prospects for a gratifying rebirth.