Why is time of service or time on job considered when recommending some for an award when it's not annotated in AR 600-8-22?

I would say it is about cumulative impact and up to the discretion of the approving authority. All things equal the longer you have been there the greater impact you will have had, since you have had more time to make accomplishments. Also in my opinion performing at a high level is our job and what highly dedicated people do regardless of awards. Therefore, doing that for a short time period doesn't merit an award or a very low level one at that. I have seen this apply not only to enlisted Soldiers, but also officers. A commander I know had her MSM downgraded as she did not complete two years of command and the CG felt her achievement did not warrant it. I don't have the reg in front of me, but I believe the MSM actually stated at one point meritorious service over a period of time, but time is relative.

never saw that...
but i used to see folks getting lesser awards because they did not already have them ... ie
get and AAM instead of ArCom because they did not have AAM...
get an ArCom instead of MSM because they did not have ArCom...
get an MSM instead of LOM and so on...

If reporting seniors are doing their jobs right, they should be parsing their commands into 3 groups for OER/Fitrep type stuff. That's the 1/3 you want to push through hard, 1/3rd you do not want to harm, and 1/3rd into the bottom group. So if you are a top 1/3rd, the time period matches with the grading and narrative on the NCOER etc. that the promotion board is looking for. You want to be picked up in the first crunch, second at the latest. Third crunch is danger close and they're looking more for reasons to toss and no longer at awards. Problem today is the fruit salad mania with EOT awards that boards now look at not having one as a red flag. So it's best that any award "define the time". It helps paint a clearer picture. On the Reserve side, we'd have people who'd expect an award for doing two weeks AD and whine about not getting one all while being mediocre doing their inactive stuff. Great fit for the bottom 1/3rd.