why does my cat bite my legs?

If it is biting your leg while you are walking, then it is acting out its hunting instincts. Kittens play to sharpen their instincts.

Because you let it!

your cat is playing with u. play with him. when my cat was a kitten he liked to climb my leg. and he did not care whether I was wearing a skirt or trousers. he stopped doing it when he grew older

They all have their ways like people do, if people can act bonkers so can they. I had cats that let you pick them up, others didn't like it, cats that played and others didn't, violent and non violent cats. The only thing universal with them is that they think they are supreme beings and are above humans, technically they don't need us and can live outside even though we make them live longer than they would on their own.

I wish I knew.  Mine does the same.  Mostly first thing in the morning, she tackles me as I get up and walk across the bedroom.  I have the scars on my legs.  I did wear wellies as soon as I got up for a week but she still continued.  I have tried distraction techniques and picking her up and carrying her downstairs, but as soon as I put her down, she goes for me again.  Apart from that, she is purry and loving.  I think some cats just have a mean streak.