Not sure I can pinpoint where it is explicitly stated but same concept of time value of money.
1 dollar today is worth more than 1 dollar a year from now because I can take that 1 dollar today and invest it. If I can put it into something that returns say 10% then a year from now it will be worth $1.10 which is more than the 1 dollar I get next year.
Likewise, if I have two identical players (Johnson - eligible for the draft in 2022 and Mike - eligible for 2023), Johnson will gain NFL experience and hopefully inch closer to reaching his full potential. Mike will improve too but not as much because college does not provide the level of develop the NFL does. So let's say Johnson improves by 30% this year while Mike only improved 15%. Next year Mike is drafted but he and Johnson are no longer identical because Mike is now worth 115% of his 2022 self but Johsnon is worth 130% of his 2022 self because you got him a year early and thus was able to invest more into him.
So that is why a draft pick today is worth more than a draft pick next year. You get to invest the 2022 draft pick for a year which all things being equal will mean that draft pick will have progressed further than a similar player not drafted until 2023.
I think it's more than that. If only that, it could be offset by special circumstances such as a weak 2022 draft class (in the teams position of need) compared to a stronger 2023 draft class.
Your example also leaves out the fact the the player that created additional value because of experience is also one year closer to no longer being on his rookie contract.
So there are most certainly mitigating factors that can offset your example.
I think it has a much to do with the uncertainty, of trading a known commodity for an unknow commodity, in which case you depreciate the value of the unknown.
Would the Houston Texans trade their 2023 first round pick to the Cincinnati Bengals, for their 2022 first round pick?
I think Cincy would do it in a heart beat.
In this particular case, if they just "swapped" 1sts with the Bears, and the Bears performed better than expectations, the Giants are screwed, If they knew for a fact that the Bears pick would be a top 3 pick, that changes things considerably, but there is no way of knowing that.