They are though lol. Oda is having Sanji fight battles after battles to showcase his growth. Sanji after full recovery by Zou is stronger than his PH and Dressrosa self. Then with the advancements made with his new suit he gets a powerboost heading into Wano, which Oda shows by having him clash with Drake and doing various things that highlight his growth with the suit. Then we move on to Onigashima and see him unlock powers and such. That's Oda showcasing Sanji's powerlevel progression growth.
Smoker once he enters post-TS, Oda has him face Law/Luffy/Vergo/Doflamingo all consecutively to showcase his various skills/powers, and putting on verge of death twice in that arc alone to extents that Koby has yet to experience, which triggers Haki growth. Not to mention he has a DF, which allows easier access for growth in terms of "Asspull" due to awakening/new techniques and etc.
Lucci once he enters post-TS, first thing Oda has him do is overcome Sentomaru's defense + have a clash on par with G5 luffy + then go on to fight Zoro... again being pushed to brinks of near death... that Koby has yet to experience... allowing for growth triggers for Haki. And again has a DF.
Kidd once he enters post-TS, is literally faced with Kaido and put down in a defeat, he then is faced with Kaido again, and then with Big Mom and then Shanks... pushed to near brink of death twice in the series.
Explain to me how fighting against Kuja fodder and getting bloodies, then fighting against Hachinuso fodders and needing to team up just to throw 1 attack on a giant rock arm... is the same as what the above are doing?
He doesn't. Smoker and Vergo have better haki feats.