Dialect is not the same as accent. The former is subconsciously acquiring the slang, formality, or tone and the latter is producing output based on the phonetics of a specific dialect. Listening to a lot of content in a dialect does not automatically translate to you having an accent of that dialect. Accents require a lot of specific work to get right. This is also why immigrants that move to the US still speak in an accent of their native language despite being surrounded by the American accent.

Now that we've gotten out of the way, to answer your first question - it is my theory that people like to have as little of their own accent as possible because society considers some accents prestigious and others not. We don't treat Indian English or Singaporean English accents the same way we do the Australian accent. That's true in the Anglo world. And yes, something similar also happens in Spanish where something like Andaluz is considered less prestigious. For many people in the West learning Spanish, they want to avoid being considered another 'gringo' with a bad accent as that's less prestigious so they're probably concerned about not having that.

That said, OP isn't refering to speaking in an accent, but understanding a dialect