We know that boys and girls can both do lots of the same things - boys and girls are both smart, sporty, creative, adventurous, kind etc. So we know that in reality boys are not more than girls, or better than girls. I am interpreting your question as, “even though boys and girls can do lots of the same things equally as well, why are girls thought to be less than boys?”
This is known as gender inequality, and it started many years ago when things were very different in society. To give you a very brief snapshot; historically, females were considered the nurturers, the bearers of children, and the ‘homemakers’, and men were the providers, considered the head of the family. Over time as technology and science evolved, so did society and gender roles. Along with the advancements, such as the contraception pill for women, childcare, and education, women entered the workforce and began challenging traditional gender roles which meant that men were doing things that only women used to do, and women were doing things that only men used to do.
Our society today recognises that girls and boys are both very capable and can do lots of the same things equally as well, however because our society had made decisions about how girls and boys should be, for example “boys are more sporty and never cry” and “girls should wear dresses and always present themselves nicely” sometimes boys and girls are not considered equal, even though they are. When this happens sometimes boys and girls are not given the same opportunities.
It's important that you know that as a girl, or a boy you should be given the same opportunities and that you should not be left out or discriminated against because you are a girl/boy. If you are in this situation, its important you talk to your parents or teachers about it to see if you can be included in the activity.