Why are computers allowed to make trades in a stock market regime that's regulated to be operated by humans?

External question — Personal Finance Stack Exchange Author: Ropstah Original post: https://money.stackexchange.com/questions/127476 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. This is my first question and I'm very unexperienced in the field of stock trading so please be gentle. Looking at a stock price chart (TSLA) I can see there is a so called "after market" in which trading is not possible but where the price does fluctuate. I'm assuming this is based on the fact that there's still the possibility for orders to be created in some form. It seems to me that the "closing of stock markets"-mechanism introduces the abilities for unhealthy competition practices. Questions like these arise (my actual question is #3, however the answers on the previous questions feed into that): Why does the stock price fluctuate when the market is closed? Why do stock markets close every day? ( other answer ) Why are computers allowed to make trades?