The quality is not guaranteed. Many of the DAB stations in the UK are broadcast at such a low bitrate they are noticeably lower quality than good quality FM reception.
Almost everyone wants sub streams so they can offer more options.
Never waste a bit on quality that could be delivering choice.
Subsequently rendered redundant by Napster, but we are stuck with DAB.
It's quantity now, with "just goos enough" translating to what people will tolerate.
Thought experiment for you:
Which do you prefer?
Low quality, but compelling. Or high quality, not so compelling?
That answer is what drives everything. People will take compelling every time. Very few will take quality as a primary factor.
More choice = more opportunity to be more compelling = higher AD rates = more revenue per bit.