I don't know what JTD's plans are, but he'll be 23.5 or 24.5 when he tries the NFL in the fall of 2023 or 2024. And QB is a position where you can play a long time (assuming you're good enough of course). So if he stays at WVU next year, I don't think not getting your NFL chance until age 24.5 is a big deal. I can see why a good RB would say, I gotta try the NFL now because the clock is ticking, but a QB can wait an extra year if he thinks it helps.
Speaking of RBs, I see that Leddie Brown made it to the final cut with the Chargers. Close but no cigar. I don't know if he still has a realistic chance to catch on with someone else or not.
ETA: I just looked up Stetson Bennett, the UGa QB that played ahead of JTD part of the season last year, including the National Championship game. He turned 24 during last season. He's playing again at UGa again this year and he turns 25 in October.