Well, that also speaks to a really rough history of first-round picks, especially from 2003 to 2018, with lots of other fallow periods too.
But overall it's just a quirk, really. Rarely is a player's development timeline such that he is in the starting line-up on Opening Day as a rookie with zero career ABs. First-round or otherwise, most players either got a short call-up at other times or were being held back for service time reasons (or, with the World Series era teams, frustrating veteran blocks).
Plus you have a lot of pitchers. And trades (four of the Phillies' five picks between 2004 and 2007 - one year they had a comp pick too - were traded).
The list of rookies who did debut with the Phillies on Opening Day: Stassi (PH), Herrera (slightly surprising but he had never gotten out of AA with Texas), Galvis (injury sub IIRC), three pitchers, Iorg, Denny Doyle and Bowa.
In 2018 there were games in which three rookies started but Crawford and Alfaro had already played in 2017 and Kingery didn't get the Opening Day start (nor did Alfaro for that matter - and Knapp had already used up his Rookie status in 2017).