Could be the Midgar area is so mako rich to allow this, or it allows it so it's so mako rich. Chicken, Egg, whatever.
I'll likely be consuming theorycrafting videos for a while here, but my overall is to not overthink things. And to note this isn't an "infinite multiverse" scenario. It seems like a new world is created when fate is defied, but most of those worlds die off shortly afterwards. There's no TVA to prune them, they just seem to instantly die. So there are likely only a handful of worlds at any time, and so far there are at least 3 different worlds (determined by the breed of Stamp) but some of those worlds have likely already died. For example Zack created two new worlds based on his choice to go after Hojo or go after Biggs, but he probably died in each of those and as such those worlds likely both died off. Which resulted in him going to the church where he met Aerith for the final battle. So that may already take us back to only two worlds, the one where Aerith is alive and main Aerith took over the body and is now with Zack, and then our main world. But I may need to rewatch some of the scenes because Sephiroth was already in those Church scenes, but I think Aerith was only there with Cloud and that was possibly a 3rd world not the Zack+Aerith offshoot. But I also haven't played any Crisis games so the theorycrafting videos also catch me up to the lore I missed in all of those, so it's been neat.