That's because the solution is straightforward, the people who built it are smart, had years to do it.
If I can build the basic scalable submission and comment system in one 8 hour day I am sure they can fix it in less than that.
Then you need to iterate of course and get the bugs out. But come on. I am incredulous that HN is buckling under a load of 20 stories with on average 100 comments each. Seriously?
The solution isn't straightforward because the problem isn't straightforward. It just looks like it is, which is by design.
The problem isn't straightforward because the software does a ton of things to try to keep the site good. We do a lot of manual moderation too, but anything we can write code for, we of course do.
It can't be any harder than not coming off blithely condescending.