We do know that SSRIs cause a withdrawal of sorts if stopped abruptly, this implies some sort of adaptation in the brain's serotonin system, or some other downstream effect. To really answer why you don't generally grow resistant to SSRIs would require to understand exactly why SSRIs work for depression, and we don't yet. One hypothesis is not that the additional serotonin is the direct way they work, but that the presence of more serotonin downregulates certain serotonin receptors and that downregulation is ultimately what gives it therapeutic benefit.