Good question!
I think one basic reason is that hero vs. hero is super easy – there’s a reason that went Marvel started the big line-wide crossover thing with Secret Wars, they went with the simplest possible story where everyone gets teleported to an arena and told to punch everyone. You don’t need backstory, you don’t need interesting motivations or a good setup, you just smash action figures together (literally, in the case of Secret Wars, which was created at the behest of Mattel to get them to do a series of Marvel figs, and Mattel’s focus groups decided everything from the name to what characters would look like) for 12 issues and call it a day.
Another basic reason is that editorial and publishing focuses a lot on trying to replicate past successes – sometimes this works ok, and sometimes this doesn’t. I would argue that Infinity Gauntlet and Infinity War build on one another nicely, but it’s patently clear that the only reason Civil War II happened is because Civil War sold despite Civil War’s garbage quality, so surely Civil War II will sell as well? Turns out not so much. So it’s a bit like movie studios trying to chase past trends instead of understanding what underlying features made those trends popular.
A third basic reason is that, for a long time (up until Time Runs Out/Secret Wars) Marvel didn’t really do line-wide continuity reboots that require crossovers, especially in comparison to D.C. who did a lot of these, primarily to “solve” various problems with alternate Earths and timelines, conflicting character identities and backstories, and the like. While Marvel did have alternate Earths/realities, it didn’t go in for them nearly as much as D.C did and historically it was perfectly comfortable leaving those alternate scenarios as What Ifs? or cordoned off in their own times, as opposed to trying to bring everything together into one Earth/universe. So if you don’t have that as a guiding principle for the story and your genre’s fundamental mode of expression is action, you can see why people keep reaching for “Who Would Win in a Fight?”