I teach children English for a living, but adults part-time. If I could switch those two jobs, I would do it in a heartbeat! Teaching adults is 100X better!

what's an adult?, we are scared to talk, kids don't give a hoot. but when (and that's a long time) we do, our convos are way deeper, Depends on the language too, if it happens to be easier then the learning get compressed vs a more complex one like Russian, given the identical effort/enthusiasm.

It's said that children are like sponges! Can learn faster than adults.

Because adults are busy and have a life. Most likely the adults that you have don't study or find it fun. Also, an adult's belief and/or experience might make it hard for them to change mindsets.

Children learn languages much faster than adults can because their brains are still in the developmental stages. Adults have already learned a language and have been using it for a long time while children are still learning and are able to more easily grasp a second language.