Because the D1 guys got recruited for their mile-5K times, which may not be great predictors of future marathon success. Then they spent 4 years training for distances from mile-10K with lots of hard, fast reps and tempos, which isn't great marathon prep.
Conner Mantz was able to make the transition fairly quickly, but a) he had a full year to transition between 2021 XC and the 2022 Chicago Marathon, b) he didn't run any track races between the 2 -- he ran 2 half-marathons instead, c) he was already 25 (2 months shy of 26) when he ran Chicago in 2022, so his body was more mature than the typical recent college grad.