What is the reason I find a high percentage of companion animals, both dogs and cats, harboring a human lost soul spirit attachment?

As you have seen intuitively, this is part of their mission. They are designed and tasked with the goal of helping human in all ways. They are able to see much intuitively, humans cannot. This includes spirits. When they see a human spirit struggling and afraid, they will seek it out to give comfort as they will for a troubled human in the physical. This is all the troubled spirit needs to form a bond and to accept a welcome, and then the energies will merge and the attachment will continue as the animal will not reject them but will nurture and protect them indefinitely. So this, in effect, is a cleanup team to scour the landscape and attract the lost ones, to give them safe harbor, and in the energy of a companion animal they will be safe from outside spirit attacks that otherwise would overwhelm them most likely and give them a severe time of torment. So this is one of the highest and most valuable of services these animals perform. As you know, the pain can be tremendous for a lost soul spirit in limbo set upon by the spirit meddlers who wish to crush them and drain them of all energy. So this is a noble duty and a divine duty they perform.