It would have been impressive for Apple to use USB-C before it was designed.
USB-C connector designed by Intel and Apple(!):
2012
USB-C spec released:
August 11 2014
12" MacBook (with USB-C) released:
April 10th 2015.
Magic Trackpad 2, Magic Keyboard and Magic Mouse 2 released:
October 13 2015.
Magic Keyboard with Touch ID - released alongside the USB-C-only iMac in 2021
MacBook Pros with TB3/USB-C released: October 2016
So not only was USB-C
available when the current Magic accessories launched, Apple had already released one USB-C Mac and a bunch of adapters with USB-C, were involved in the
development of USB-C a couple of years before, and were almost certainly already planning moving the entire MacBook range over to USB-C/TB3.
Now, OK, the use of lightning on these accessories doesn't keep me awake at night (they come with the required cable, and it's only the Magic Trackpad I've got any respect for anyhow) - but it is perplexing why these accessories ever had Lightning when Apple was already rolling out USB-C to the Mac.
I think if Apple was planning to swap from Lightning to USB-C for the Magic Keyboard, Mouse and Trackpad they would have done so when the M3 iMac was released.
I suspect - elaborate conspiracy theories notwithstanding - it's just a case that none of these are big money spinners, and punters only buy them if they've already decided to buy a Mac, so Apple won't lay out one penny more than they have to on updating them. Things like needing a Lightning cable (which is included anyway) and not being able to charge the mouse in use aren't dealbreakers, they just fall under "could do better" from a company that once made a $300 coffee table book about what wonderful designers they were.