Processing systems of search engines are complicated and secrets, which make it hard to make necessary changes for better positions on the list.
Information processing systems change all the time. Therefore, after changing anything in a website, information could be lost when filtering system is changed.
The spider software of each search engines are different.
Each adaptation or optimization could cause a website to have a lower rank.
The spider software periodically inspects its website. Each website requires weeks to update their information.
SEO takes more time and is more expensive, but has less rate of success