https://www.scientificamerican.com/article/howcan-a-poll-of-only-100/
The margin of error is a mathematical abstraction, and there are a number of reasons why actual errors in surveys are larger. Even with random sampling, people in the population have unequal probabilities of inclusion in the survey. For instance, if you don't have a telephone, you won't be in the survey, but if you have two phone lines, you have two chances to be included.