They do but the other Androids had bigger roles honestly; 17 and 18 decimated Future Trunk's timeline, and in the present not only presented that threat but did more. 16 was the one who triggered Gohan's SSJ 2 transformation, and was a good contrast to the other Androids while being able to hold his own against Imperfect Cell. 17 awakened 16 and even got two people to give energy to Goku's spirit bomb, while also being absorbed into Semi-perfect Cell and used against 18; I think 17 does more in the History of Trunks special too, to Future Gohan anyway. 18 is Krillin's wife, beat the hell out of Vegeta and Trunks, was the key focus of Semi-perfect Cell and why Tien got involved and why 16 had to forfeit.
19 and 20 just appear for those few episodes, and to me, don't do too much memorable outside of showing the messed up timeline. You're right that they're important, but not to the extent of these Androids; the lack of episodes might be part of it too. Plus 20 had a body shape like Majin Buu, and we don't know how Bandai can handle that. I know it's been proven that it can be done, but Bandai might not know.