The short answer: Because their parents did.
Pretty much world wide i would think.
Spanish in Spain is not exactly the same as Spanish in Mexico. Spain has an extra conjugation tense for verbs than does Mexico. And even within Spain, Spanish is different. I lived in southern Spain in Andalusia for three years as a kid. They were known for using slang and cutting off the ends of their words. Northern Spaniards speak "Castilian" Spanish which is more proper and formal. Similar to the twangs and drawls in the southern US versus the formal pronunciations in Boston.
It doesn't take long living in a new part of the country to pick up the idiosyncrasies of the words and language from that region. When my kids were young, we lived in South Carolina for a while. They quickly started talking like their peers at school and developed southern drawls. They lost the drawls over time when we moved back to New Mexico and Arizona. Even as adults, it is hard not to adapt the local tongue that is spoken around you. When I met my wife in the Air Force in New Mexico 40 years ago, she had a distinct southern accent to her speech because she was raised in the South. It softened over the years but I could still pick a little of it up in accent and words over the years, even though we didn't live in the south. She still uses words that are distinctly southern. She says "mash the gas pedal" instead of step on the gas. Or she calls a coke a soda pop. And my pants are not pants but "britches". "Nabs" are peanut butter crackers. And sometimes it is hard to distinguish what word she is using when she says "all". It is "all" the people in the world or does she mean the "all" as in "oil" for the car. They both sound the same to me.
But we are living in Florida now and even I (who was raised in the western US) have picked up a slight exaggerated southern drawl because is is hard not to. It is how people around you speak. And if you are going to assimilate, you will unconsciously begin to speak that way too.
Y'all come back now, 'ya hear?
Years ago I was sailing on the Chesapeake Bay with an English girlfriend and visited Smith Island. She said the locals had a Dorset accent. The locals were probably descendants of the original Jamestown colony.
The Smith islanders called people on Tangier (a neighboring island) “cheese-eaters” because they said they spoke like they had a mouthful of cheese.