Why? Because they are. The whole idea of “girl stuff” and “boy stuff,” particularly in early childhood, is ridiculous. Kids like to play, and the girls who like to play dolls probably like catching frogs and making mud pies too.
Don’t worry about “playing with girls.” Just play with friends, some of whom might be girls. No one likes to be singled out because of their gender.
Encourage them to think about how they’d feel if they started playing with a group of girls at the park who kept making remarks about the novelty of playing with a boy. It wouldn’t be fun, would it? When boys treat girls like normal friends, girls are more likely to stick around.
This applies as kids get older too. While the gap between genders grows with age, spending time with good friends who share your interests isn’t a gendered activity. If someone likes the same stuff as you do, be it video games, soccer, writing screenplays, or going to the beach, try to befriend them, girl or not.