Speculating about when iOS updates occur is frequently tied to parallel Apple OS beta seeds with aligned feature enhancements. Just consider it another way to read tea leaves of when things drop. Some years the OS's are fairly close, this year looks like it is staggered again because of all the new or changed features.
I don’t think the “new or changed features” have anything to do with the concurrent or staggered release dates of iOS and MacOS.
If there is one date in this whole process that is absolutely fixed, the expected iPhone ship date is it. And iOS 17
must be ready to install on said phones when they start rolling off the assembly lines.
Why is this?
Apple needs to record some (meaning “tens of millions”) iPhone sales during its fiscal Q4, the last 2 weeks of which follow the anticipated iPhone release date. If it does not deliver tens of millions of new phones in that period, the stock tanks because the year over year comparison to Q4s in which it has successfully released the iPhone will be, shall we say, extremely unfavorable (on the order of tens of billions of dollars in missed revenue).
As a result, all hands are almost certainly on deck to finish any necessary coding to get iOS out the door on time, including engineers who typically are responsible for MacOS. Thus, MacOS is given a much lower priority for several weeks (and that probably is during the July-August time frame, ending about now, I’d estimate).
As
@Ansath has noted, it’s been several (5 or 6) years since what is now MacOS has been released simultaneously with iOS (even iPadOS has seen some date slippage). This is why.