And you use those extra slots for what, exactly? Or, more specifically, if you have a use for more than 1 GPU as the only card in the system then you have an above-standard use case, and ITX isn't a consideration. Same goes for 4 DIMMs. And this is the point, generally-speaking a standard use case would be 1 GPU, 2 DIMMs and a couple of SATA drives or 1 NVMe drive. Anything bigger than ITX is a waste of space.
And audio is what you pay for. The cheaper ITX boards can have rubbish onboard just in the same way as cheaper ATX boards can have rubbish onboard. But look at the Strix ITX line for example with its dedicated sound card. But again, if you're picky about your audio quality then you wouldn't look at any old onboard, you'd know your stuff and find out which onboard comes with which mobo, and if you're really discerning to extent you want dedicated sound then ITX isn't a consideration.