The seat inventory is pretty much automatized - as you probably know, upgrade and award seats come out of different seat inventories. Upgrade inventory tend to be available when the seats are fist released, and as certain other inventories are taken and the RM software uses predictive analytics to "guess" how many seats will be purchased as the bottom buckets get taken.

Eventually, the upgrade seats will be taken up - and new inventory will only be released much closer to departure, when it appears the remaining seats look like they will definitely not be sold. The idea is to sell as many seats as possible (particularly in the high fare brackets, to those who need to travel at the last minute.) If they give everything away for awards and upgrades, they miss out on the big buck fare buyers - tactically and strategically stupid.

Looking in mid-April for upgrade seats on popular flights for mid-May (at least shoulder, if not high, season for many destinations,) I can imagine you would not find much availability.

Seat charts, of course, merely reflect which seats have been assigned - you could have a group of thirty who are ticketed and do not have seat assignments (e.g. tours.)