For me, because they're enjoyable! I find armed conflict far less interesting in the older games. Despite you finding it jarring and aggravating, that clearly hasn't impacted sales of Civ 5 and 6, which are the largest in the series - I can't state conclusively that people enjoy 1UPT more than unlimited stacking, but the presentation of 1UPT as something antithetical to the series seems strange to me when the majority of the players of the Civ series started with a game that uses 1UPT (and probably haven't played any other version of Civ). We're definitely a bubble here on these forums of people who have more experience with the older games. As Bonyduck says, so much of Civ is wildly out of scale - it takes centuries to conquer a single city in the earlier eras, the first voyage to circumnavigate the globe normally takes centuries, and by the time you'd have built up an army of one troop and got them to the front they're probably getting close to being outdated technology, for example. There are plenty of abstractions in the series because they lead to more engaging gameplay, and I do not mind limited stacking being one of them. I'm happy for there to be some movement towards limited stacking too, so long as the fun of the 1UPT combat system remains.