There's no reason to overanalyze things. Inzaghi just doesn't play youngsters. It's that simple. And it isn't an issue that started at Inter. It has always been an issue since the start of his coaching career. Here's an article from 2021 at a Lazio fansite:
Lazio coach Simone Inzaghi has earnt praise from the Biancocelesti faithful for his impressive work over the last five years. The 45-year-old Italian...
thelaziali.com
It's not a coincidence his squads are always some of the oldest in the league. He inherited the 3rd youngest squad in Serie A when he took over at Lazio in 2016/17. Take a look how it progressed in the following years:
Season | Average Age | Serie A Average Age Rank |
2016/17 | 25.5 | 3rd |
2017/18 | 26.5 | 9th |
2018/19 | 27.6 | 14th |
2019/20 | 27.7 | 16th |
2020/21 | 29 | 20th |
At Inter he already inherited an old squad (3rd oldest at the time - 28 average age) but we have become even older despite offloading Perisic, Alexis, Vidal, Kolarov and Ranocchia (28.9 in 2021/22, 28.3 in 2022/23 - both ranked at 20th).
As long as Inzaghi is the coach youngsters won't play at Inter.