In principle I agree. Why not have the sound bar include Apple TV in it etc and then why not include a sub woofer and on and on and on.
However, context. Sonos is a company that is worth US$1.4bln and Apple is worth US$3.4Trn - Apple does sales that is 280x that of Sonos - Apple makes more revenue in 1-2 days than Sonos makes in an entire year. Apple could buy Sonos and its like a few days cash flow. This segment is just too small for them. Everything Apple sells is about getting you to keep buying iPhones (yes that includes Apple TV, Airpods, Macs etc). I think the sound bar market is too small and niche for them and I don't see people buying iPhones because of a sound bar etc. That's not to say I wouldn't mind it happening.