Muslims around the world follow the teachings (Sunnah) of Prophet Muhammad and the teachings in the Quran. The Sunnah of Prophet Muhammad (peace be upon him) also mentions the rules for drinking water, one of which is to drink water while sitting.
The biggest reason why Muslims drink water while sitting is the Sunnah of Prophet Muhammad. Drinking water while sitting is considered Sunnah in Islam. On the other hand, drinking water while standing is considered disliked (makruh) in Islam and sitting is considered a better way.
Prophet Muhammad himself used to drink water while sitting and since then drinking water while sitting is considered Sunnah. At the same time, science also believes that drinking water while standing can cause many health problems like acidity, pressure on the kidneys and joint pain etc.
There are 6 Sunnahs for drinking water in Islam: 1) Say "Bismillah" before drinking water, 2) drink with the right hand, 3) drink while sitting, 4) look at the water before drinking it, 5) drink water in three sips, and 6) say "Alhamdulillah" after drinking water.
Muslims do not drink all the water at once, but drink it in three sips with breaks in between. This rule is also related to science. Many studies have shown that drinking water at once without stopping damages muscles and nerves.
In Islam, it is necessary to say "Bismillah" before drinking water. It is said to always use the right hand to drink water. Prophet Muhammad's message on this is that do not eat and drink with the left hand, because devils eat and drink with the left hand.
However, in Islam, water can be drunk while standing under certain circumstances. Prophet Muhammad used to drink Zam Zam water while standing. Since then, all Muslims drink Zam Zam water while standing. Some people believe that drinking Zam Zam water while standing spreads its blessings to every part of the body.