Your 1080p blurays most likely a higher video and audio bitrate than the 4k that itunes puts out.
For video I'm going to have to disagree with the implication that 1080p BD is better than iTunes 4K because of higher bitrate. Comparing the bitrate of two different video codecs is outright misleading. Higher bitrates in older formats actually just mean inefficiency. Examples:
- MPEG-2 needs 60% more bitrate to equal picture quality of AVC
- HEVC only needs 50-70% of the bitrate of AVC and has better PQ at that lower rate
Notably, there are several comparisons of 4K iTunes titles against 4K UHD discs, and there is precious little difference. (And note, the disc is an UHD disc, not even a 1080P disc.) If we are at the point where we have to closely look at single captured frames for a few seconds or more in order to determine the very tiny differences, then I'd say we are close enough.
I do agree with you on audio. Audio on streaming is definitely worse than discs. This is noticeable even with my low-end surround setup ($300 for six speakers, or $50 per speaker average).
But even so, streaming audio is improving fast. For example ATMOS is coming to Netflix, Amazon, iTunes, and VUDU.