Might not see a lot of kids wearing Eovaldi jerseys, but you can be sure Nate's a star alright with teammates of his three postseason clubs: '18, '21 Sox, '23 Rangers.
But kids are relevant in today's discussion, because growing up rooting for favorite players is a big part of baseball fandom, including shirts, posters, cards, autographs, etc.
Fans get attached to stars, and some semblance of roster continuity is important, even in the era of free agency. It's obviously not the same as last century, when families were on first-name basis with the same starting line-ups being described on their radios for a decade or more on some clubs (ex: Brooklyn NL mid-1940-50s, Detroit AL 1960s).
For the most part, we all want to cheer for teams of players who at least seem like they care about winning as much as we do. And fans definitely care about good groups that fall short or don't even reach the last game of any season. The human element makes us empathetic to our heroes, and in some cases, root even harder for them the next time...
... if only the owners pay them what they're worth to keep wearing our laundry.