It's obvious generally and as a subtext of your own comment that it
isn't "so easy".
It seems more productive to look at just what it is that gives Reddit value and is of general benefit.
There are and have been Reddit contenders, among them Tildes (of which I'm a member), the former Imzy (which I joined, and watched implode spectacularly), Lemmy (which I've known of vaguely for some months) and kbin (which I've only just heard of), as well as, of course, Hacker News itself (and clones such as lobste.rs), and extant fora such as Slashdot, Digg, Fark, and the like. Not to mention Facebook with its own Groups features.
The problem facing most extant fora is their own reputation (or lack thereof). Digg, Fark, and Slashdot all burned through goodwill and interest. I was a ... very early ... participant in Slashdot, let's say, and don't think I've logged in for years, though I'll very occasionally peruse the site. The story selection may be OK (it's often comparable to HN). The discussion level is ... poor. In the extreme.
I have less experience with Digg and Fark though I suspect the general sense is similar. Both had the spark for a time, but that torch was passed. Fallen online cultures virtually never revive.
Imzy set out as its goal, "a kinder, gentler Reddit", solving problems Reddit expressed at scale, without ever first solving the problem of reaching that scale. Imzy also managed to create a few pathologies of its own, a key aspect of which was centered around its own serial pseudonymity. HN's policy of permitting pseudonymous accounts, even temporary throways for specific discussion, but banning repeatedly and frequently generating new accounts seems to me well-founded and a good balance. Reddit has effectively had a similar policy, largely through karma requirements which make brand-new low-karma profiles of limited use, though reputation can be built over a reasonable period of time.
Tildes has existed and operated reasonably well for about seven years, being launched in 2016, and I'd joined it quite near that date. It's avoided Imzy's pathologies, but has a general scaling problem, and discussion tends to be both thin and not particularly specialised. Reddit's advantage for now is that there are corners with deep technical knowledge. Tildes might get there, though I'm not sure that's its goal. For now it's ... a reasonable option.
Lobste.rs is nearly identical to Hacker News as regards Reddit, except smaller. There are no sub-discussions, there is now individual ownership of such discussions, and effectively the entire site is /r/all, much as HN is (as I recently noted: <https://news.ycombinator.com/item?id=36256208>). As with Tildes, Lobste.rs has functioned usefully for some years, it's not been a runaway success, and it seems to fill a different niche than Reddit's topic-based user-moderated forums.
Site growth turns out to be a hard problem. Getting a small department to agree on a lunch spot, let alone several tens of millions of Internet strangers on where to decamp to is a large coordination problem. (I've been through this myself, several times, notably with the Google+ shutdown.) Successful migrations typically take both time and significant advanced planning.
And that's mostly looking at the scaling issue.
I've argued as well recently that Reddit provides a huge amount of behind-the-scenes moderation and filtering, and that this is likely a large part of the site's value. <https://news.ycombinator.com/item?id=36332367> The immediate effects of, say, a widespread user-based moderation strike would probably NOT be overwhelming amounts of spam and pr0n within subreddits. Most of that is already caught by automated tools.
Instead, what you'd see would be less egregious, but equally corrosive drift and devaluation. Off-topic posts, below-threholds-of-automated-detection levels of incivility, bullying, and trolling, shallow content, and the like. For some subreddits that's not especially significant, for others, such as AskScience and AskHistorians, which are extremely heavily moderated (I've in fact refused further participation in the latter based on moderating out of SciHub and related links), but which would devolve to chaos or irrelevance without such human intervention. If that's a small number of subs and Reddit finds them sufficiently valuable they might be able to substitute professional paid mods, but doing so for even a small fraction of the 100+ million "active" subreddits the site boasts ... would not scale.
Still, the automated infrastructure represents a huge amount of institutional knowledge and goodwill which upstart contenders would have to replicate.
Again: just two elements of the problem, neither of them easy. Not impossible to solve, but chance (and hence risk) plays a huge role.