When exporting a webflow project code the export adds a .HTML to every URL throughout your site.
After years of coming across this issue and using the laborious workarounds to fix this post-export, it really boils down to Webflow just wanting to make things difficult because you're not using their hosting. The Co-Founder in a 2013 thread on this very topic says it's because exported sites don't work on local machines without file extensions. He says this is because some folk won't be savvy enough to set up a local server.
This is good and all, but I'd be willing to bet that 90% of people exporting code are just immediately uploading it to another host in some capacity - which I would argue is the less-savvy route - and they do not need the .html extension. If people are exporting the code to edit themselves on a local machine, they're probably savvy enough to not only set up a local server, but also to add file-extensions post-export to make it work. The funny thing is, all of the solutions to this issue on the forums from people asking how to remove the .HTML from their exported code (lots of them) require you to be relatively technically savvy anyways.
All in all, I just think it's an incredibly arbitrary feature that has 6+ forums dating back to 2013 with people confused as to why this is a thing. You'd think they'd implement some toggle feature to choose if you need file extensions or not. Or to export individual files. Or something just obvious like this. But that's where we're at. Not hosting with us? Eh deal with it.