From a practical standpoint, Steven Yeun at age 34 can’t pass for Steven Yeun at age 25, when he swooned on set.
Other actors — including Andrew Lincoln and possibly Jon Bernthal in Season 9 — played flashback versions of themselves for the show, but filming a flashback to young Glenn is apparently not something that Yeun is interested in.
It would be downright strange to see Yeun try to play Glenn the pizza man again.
That Glenn was long gone anyway by the time he was killed by Negan.