Why don’t we take precaution in other cases, such as voluntary (mustahab) fasting?

Answer

In the name of Allah, as for voluntary fasting, the Sharia allows a person to intend fasting even during the daytime, even after midday, provided they have not eaten anything that breaks the fast. This is a concession from the Sharia for those who wish to observe voluntary fasting, and the ruling is purely an act of worship. Allah is the Guide and Knower.