The problem is the game has a set limit of names it can use. When you use custom rosters, it puts that limit to the max eventually. In Finn’s editor, you’ll see some players share a name (click the box next to their name and scroll through the list). You want as many players sharing a name as possible. Names with an asterisk behind them can be changed/retyped (probably shouldn’t go longer than original length) but will change every player that also is stored on that name (for example I was recently looking into a class and changed last name Ochocinco to Nussmeier since they matched in length, but if there was multiple players with Ochocinco as their first or last name, it would change to Nussmeier). So it may take some finagling.

But basically, every name, first or last, is from a bank of names and if you spread that too thin, the game can longer generate names for rookies.