I am trying to do just that, and I'm learning "from scratch." The reason people don't bother is because of overpriced rip-offs like Rosetta Stone. They charge hundreds of dollars for a lousy language program that is little more than stale, warmed-over Berlitz. The Berlitz method is the WORST way to learn a language! And, no, it is impossible to learn a language (or anything else) by studying merely ten or twenty minutes per day. But that's what they claim. A true language student must understand basics such as grammar, word-usage and sentence structure.
In all fairness, Americans are lazy learners. They don't realize that learning anything requires commitment, as well as intensive and determined study! Two and a half hours per day is closer to reality! But my stupid country would prefer to waste time staring aimlessly at stupid-phones, or binge-watching some idiotic TV show. And look at the result: Americans don't even know the difference between a preposition ("to") and an adverb ("too"), nor do they care to learn. That's why.