why do british shows have only 3-10 episodes per season?? very confusing for this yankee haha

There's not as much money in British TV as in the US so rather than pouring all their money into 22 episode seasons it's spread around a lot more variety to give good quality production. More recently there have been joint production with bigger international corporations, but often that means that an international audience is catered for in terms of language or explanation of things which a British audience wouldn't need explaining.