Why do many people only adopt puppies/kittens rather than adult animals?

When there babies there so cute, also they want them for a long life

Because babies are much cuter than grown ups.

Well, experience. When you rescue an adult animal, what are you rescuing them from? A loving, stable home where they were taught how to be gentle and good?  Ehhhhhhhh probably not. It's very noble to bring in a rescue and sometimes they are very good pets indeed. But a lot of people don't want to take that chance. A second reason is that people want specific breed characteristics and will spend huge money to get it. And the babies are what get sold by breeders, not adults.

Adoption of adult animals is a relatively recent development in the pet world. When I was a kid, there weren't many animal shelters. People wanted baby animals they could raise.

People understand that keeping ANY animal in a cage away from it own kind and community, especially in filthy kennel conditions, (which is true of nearly ALL kennels), traumatizes the animal and makes it both more likely to get sick or injured and less 'friendly' overall. They also know that it won't live as long.

Taking babies away from their mothers and animal communities to live alone with humans is equally cruel, but it makes it easier to justify because the animal is ignorant of what it means to be free and among its own kind. 

Animals should be kept in their communities instead of being forced into ours as surrogate 'children,' 'friends,' or even worse. That is how I interact with them and build relationships with animals. I do not place individual animals on the same level with humans. I place their community on that level and interact with it. It is almost like interacting with a 'Person' who tests boundaries, sets limits, even asks primitive questions or makes requests of my 'human' community. People never get to experience this by isolating the animals and trying to make them 'human.'

They also miss out on the joy of seeing them interacting naturally with each other and other animals and watching their young greet the world with joy and amazement every spring. Very little is required of humans when we let animals do what they do naturally and nature itself does most of the work.

I have a community of feral cats living in my neighborhood that has been in my area for 317 years and can be traced back to the founder of the Township who selected them from around the world and then brought them over from England to help control rats, mice, rabbits and scavenger birds in the colonial gardens and farming communities. It never ceases to amaze me watching the newbies come in here and try to kill them off, cage them, steal their young, destroy their ability to procreate and cage them for 'adoption' out of 'liberal compassion.' They even self-righteously teach their children this.

We have 57 million 'caged' cats in America alone. Left in nature to surround us in their own communities instead, there numbers would be much smaller, (nature controls their population if humans don't interfere), our use of poisons to control vermin would be non-existent, (limiting the cancers we give ourselves), and many more of us would have actual 'human' friends and stronger communities that would enjoy watching animals be animals, instead of this 'compassionate' atrocity we have now.