Keep in mind that new to us isn’t necessarily new. We are so far from planets like Jupiter and Saturn that we are limited by our own methods of observation.
If a new moon is discovered, that doesn’t mean it appeared out of thin air. Most likely, it’s been there for some time. Though it should be noted that new moons appearing is entirely possible, and most likely inevitable.
Jupiter sits near a very large asteroid field and there’s a great possibility of yesterday’s asteroids getting sucked in by Jupiter’s massive gravitational pull, only to drop into orbit and become tomorrow’s moons.