But what evidence shows they reset every decade or so?
That's actually not completely true. Details that may not be addressed or revealed within episodes or films can still be viewed as canon. A good example of that may be details revealed in say, a Marvel Comic Guidebook, revealing a minor character's middle name or some other less than important detail. It does happen. I always assumed that was case with making Kirk's birthday same as Shatner's. It's not addressed in the episodes or films (the only canon material) when his birthday is (in Prime timeline, Kelvin timeline makes it quite clear), but I thought it was generally agreed Kirk's birthday is March 22.
It's accepted as such on Memory Alpha and various other info sites.
Presumably Roddenberry chose Shatner's actual birthday to align with Kirk's during this:
"In March 1985, when the town was looking for a theme for its annual town festival, Steve Miller, a member of the Riverside City Council who had read
The Making of Star Trek – a book that lists Kirk's year of birth as
2228 rather than
2233 as established in
TOS: "
The Deadly Years" – suggested to the council that Riverside should proclaim itself to be the future birthplace of Kirk. Miller's motion passed unanimously. The council later wrote to Roddenberry for his permission to be designated as the official birthplace of Kirk, and with Roddenberry's consent,
the towndeveloped a tourist industry around the idea.
Star Trek IV: The Voyage Home then established on screen that Kirk was from Iowa."
Presumably Roddenberry determined Kirk's birthday was same as Shatner's, or agreed to it based on someone else's suggestion.