Our bodies are made to pronounce a ton of sounds we both can't hear and speak as adults.

https://www.smithsonianmag.com/science-nature/accents-are-fo...

The expert quoted in that article disagrees with the idea that you can't do it:

"People talk about a 'window of opportunity' for learning language," Kuhl says. "The implication is that if you miss that opportunity, it's too late. I don't agree. It is more difficult with the years. But not impossible."

Personally, I think the largest difficulty is the amount of practice required. Kids can babble about all day, but as an adult, even investing an hour a day into language practice is hard to find the time for.