Baby won’t sleep without being rocked

Hi all. What can I do please? You can’t lay my 11 months old down for him to sleep because he will cry so much! He wants to be rocked all the time. He is a big baby and my arms are killing me already. Sometimes I get muscle pull, It takes 10-15minutes for him to fall asleep while being rocked. I don’t know what else to do. Why can’t they just lay down and fall asleep by themselves?