There are no hard and fast rules, only tendencies. And the tendency to devalue – which isn’t universal – is consistent with the trend towards rewarding less across the board, tying rewards to spending (while rewarding even high spend less), and then making tweaks to the less rewarding model as business needs dictate.
Business decisions made in isolation compound over time taking what was once a simple program and making it complex. Multiply that out over tens of millions of members that a given program is trying to serve and tweaks along the way get made which only add to the complexity.