Because with our luck, the fourth OF will start 60 games.
Looking at their (albeit somewhat short) histories, both Bellinger and Soto have played in a lot of games almost every year (Bellinger had a short '21). Judge, apart from ridiculous situations stays healthy.
If they want to bring a Keirmeier or someone as a 4OF
and load up on the starters, okay. But they're not bringing in him or Margot as a 4, they're bringing them in as a starter so they can save money.