I think it would be pretty easy to make a clone, but everyone who tries seems to end up getting nerdsniped making something federated instead.

Only federation defends from what is now happening with Reddit. See Freenode and Libera Chat.

Reddit was great for a decade, why can't we just switch website every 10 years?

Because you usually loose a large part of the community on the way?

I'm fine with that. Massive popularity is a poison pill.

Not only do you lose the community as already mentioned, but you lose the accumulated data (i.e. post content). This might not matter for a social community; it matters a lot if the community is of a more technical nature; i.e. repair/modding, analysis, etc. A lot of really great and useful content has been lost over the years due to forums going dark; lose a site like reddit and... my god. It's just really a huge step backward.

N.B. I have no love for reddit. I generally hate reddit, the site. However there's a massive amount of useful content there.

Why use an alternative when you can use the real thing? Reddit was open-source until 2017: https://github.com/reddit-archive/reddit

I think that qualifies at trivial.

Running things like this is not exactly trivial, even if code works run well as-is 6 years later

There are a lot of alternatives that replicate Reddit's functionality. I've tested out three or four of them over the years.

The quality differences are 100% due to the differences in the communities. They are social deficits, not technical ones.

Creating a decent Reddit clone is a weekend project. I've done similar stuff as a demo teaching an hour long 'Intro to Rails' class at the local code school.