Strawhats rarely get fights, and when they do, often they're either mostly off panel or short. Do you really think that side characters in a flashback during an event with dozens of players, while we are at the climax of the present arc, will get proper fleshed out fights?. Clashes, especially amongst top tiers, are the easiest way for Oda to showcase power levels without needing to put a definitive stamp on whose stronger, which breeds discussion among the fans. The mystique of their power still gives them gravitas, Oda want's to keep that alive with the legends, while avoiding overexposure. Also, if Oda were to actually draw a full scale epic war/showdown between all these top tiers, then it'll be difficult for him to top it with the current events which don't involve as many powerful players, it'll dwarf Elbaf's climax or at least minimize it's impact. The only war that matters is the one involving the Strawhats.