It's partly because these missions take so long to pay off. The Jovian and Saturnian moons are definitely targets for landers, as is some of the asteroid belt. There is a significant signal delay as well, meaning nothing would be done in real time.

Martian rovers can have a twenty minute signal delay one way. Meaning a return signal takes over forty minutes. If it isn't behind the Sun. By the time probes get to Jupiter, the delay on a return signal can be the best part of two hours. That can increase even more going to and from Saturn.

Increase that by even more for Uranus and Neptune.