In the case of ayahuasca, quite possibly because they're dead. You don't mix SSRIs and MAOIs unless you have a very good idea what you're doing, and even then probably not without a reasonably well-equipped clinic to hand.
Serotonin Syndrome is not something to fuck around with.
Not that I'd advise anyone take the risk, but there are lower risks associated with mixing SSRIs and MAO-B inhibitors (like ayahuasca). It's the older class of anti-depressants which are both MAO-A and MAO-B inhibitors that you don't want to mix with SSRIs (and lots of other things too). MAO-A oxidises/metabolises seratonin, noradrenaline and dopamine, whereas MAO-B is mostly just dopamine. MAO-A inhibition also causes the 'cheese effect' (consumption of tyramine causing hypertensive crisis).
As an aside, it's a real pity that the side effects of MAO-A inhibition loom so large in people's imaginations. They're extremely effective anti-depressants and many of the risks can be easily mitigated, but GPs and even shrinks are now too afraid to prescribe them.
In the case of ayahuasca, quite possibly because they're dead. You don't mix SSRIs and MAOIs unless you have a very good idea what you're doing, and even then probably not without a reasonably well-equipped clinic to hand.
Serotonin Syndrome is not something to fuck around with.