No, it isn't about marketing.

1) The requirement of the custmer is not good absolute precision. But good relative precision.

2) You need the extra bits - even when there is noise there - when you perform digital filtering and amplitude changes.

Think about it. If you do record a CD at home - a lot of groups have done that - then you can't do it with a 16-bit card. Even if it is extremely good.

You record multiple channels one-by-one in 96 or 192kHz with 20 to 24-bit resolution (but not precision).

You then might add digital filtering on the individual tracks if you want to adjust how they sound. For that, you need the extra resolution you get from the 20 to 24-bit recording.

You then have to mix the channels, and when mixing, you need to adjust their individual volumes. So you need the extra bits just to avoid discretization errors where one bit from 16-bit sound gets misaligned after the fractonal multiplication.

Once more - people who buy audio equpment are not magically interested in 24-bit. An electronics engieer might. But audio people are interested in THD, SN, ... And they know that they need to perform operations to the audio. And they know that all these steps are performed at a higher sampling rate and higher bit resolution than the 16-bit CD audio.

So while you might find it strange to buy a 24-bit card, people who work with audio won't. Some of them even write their own code to run in their audio cards, since the cards are quire capable DSP - even if the DSP acronyme is sometimes expanded as Digital Sound Processor.

Remember also that some of the cards have 96kHz, 20-bit capture and 192kHz, 24-bit playback.

But do they get CD quality? Yes, they do. They definitely do. Check the tests for some of the cards. And the relevant figures are not number of bits of of theoretical or valid bits, but the figures are measured and presented in form of frequence response, distortion, noise.

The people who buy the highend cards because they need them, knows that # of bits is about as applicable as the clock frequency of a PC processor, i.e. hardly relevant at all. Except for the signal processing stage.