For similar reasons (cost) they also use lower fidelity sound mix. This is most noticeable in the surround channels. If you have at least a 5.1 system, watch a show you own on a streaming platform and then on 4k blu ray. Even those who don’t care about such things will notice the difference. The majority do not though, or would be unwilling to stomach increased cost to get there with streaming today. It also doesn’t help that the general internet infrastructure probably can’t support it en masse.