Can u tell me why vets do this?

ok well i know this lady theres this dog she started showing up in january we hadnt seen her for awhile she comes back and she is all bones not any fat its like her body is eating its own flesh and at that she had puppies we can tell cause her are full of milk and are swollen so we are thinking ok wheres her pups but besides that she so scared of people we have to open the window on are screen door and feed her because if we open the door she will run so we call shelters and talk to the vets and they say we cant do anything when all wer asked is will they do her a favor (the dog) and come put her down and we have called so many shelters and they say the same thing why become a vet if you wont help an animal just because its inconviences you whats the point arnt vets suppose to love aanimals and be willing to do anything for them??? can someone explain to me why they would do that