Not all potential traits are going to be present in the population. If this were true nothing would go extinct as there would be enough diversity in this theoretical population to see some individuals with the right combination of traits.
It it somewhat more likely to happen when you have say a flask of bacteria where they grow logarithmically by the hour in terms of generational time and have much simpler single cell systems vs us poor multicellular well differentiated humans that are waiting until our thirties when reproductive systems start failing to have our 0.6 kids or whatever the rate is in western countries where diversity is already quite low due to a lack of significant african demography in most populations out of africa. Even in places with significant african background population numbers, social history means these alleles have not yet dispersed across the population homogeneously and are maintained in their demographic subset.
It it somewhat more likely to happen when you have say a flask of bacteria where they grow logarithmically by the hour in terms of generational time and have much simpler single cell systems vs us poor multicellular well differentiated humans that are waiting until our thirties when reproductive systems start failing to have our 0.6 kids or whatever the rate is in western countries where diversity is already quite low due to a lack of significant african demography in most populations out of africa. Even in places with significant african background population numbers, social history means these alleles have not yet dispersed across the population homogeneously and are maintained in their demographic subset.