If you have a container filled with one pure substance (such as a pure element), practically all of those molecules must be neutral (they must have no electric charge). Otherwise, the mixture would be highly unstable. Why is that?

For electric charge, we say that opposites attract and like repels like. In other words, objects with opposite electric charges (+ and -) will attract, while objects with the same electric charge (+ and +, or - and -) will push each other away. Because of this, if (for example) you could somehow fill a container with only positively-charged particles, they would immediately shoot away from each other--maybe explosively breaking their container.

On the other hand, if you put both positively-charged and negatively-charged particles into a container, they would immediately bond together to form new molecules that have neutral charge. Technically, this would be a chemical reaction between charged elements, but because it would happen so fast, it is almost impossible to set up and observe. This is why reactions that are useful to study are usually neutrally-charged.

Attraction and repulsion caused by electric charge are both examples of electrostatic forces. These electrostatic forces are usually very strong compared to other forces like gravity, so electrically-charged things tend to find oppositely-charged things very quickly and become charge neutral very quickly. This is why scientists often assume electroneutrality. Electroneutrality is an approximation that says that the total charge in a system is zero (meaning the total positive charge cancels the total negative charge). It is not exactly true--you can often have a slight imbalance in charge--but systems are usually so close to neutral that we might as well say that they are neutral.