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Not pills, but nutritionally complete drinks exist and are fairly popular.. there's a section for them in the local supermarket that didn't exist a couple of years ago.
Pills would have the same problem as anything else Economics means they'd charge the same or more (because they could) than making food anyway, making it unviable as a complete replacement for most. Or they'd make cheaper ones that were bad for you - like ready meals, you can buy quite healthy ones at a price or get total crap cheap, guess what most buy.