Karma-farming bots exist. It's trivial to make bot accounts and farm karma on meme subs until you're over the threshold, and then go and comment in product-related subreddits.

Technical solutions are extremely difficult. A verified purchase mechanism is difficult to do unless you control the purchase platform.

The "easiest" solution is a web of trust, but that has a significant non-technical component by definition.

I'm sure the reason is "too complicated and not worth it" but I've always been surprised that Reddit doesn't have per-sub karma.

It does and you can limit posters to in-sub karma. afaik the only way to implement that solution is to write a custom bot though. Reddit's tools for moderation are basically garbage.

The Reddit karma algorithm is already a black box, so there's not much harm in making it more complex, thus:

Measure the general up-votiness of each sub, and scale cross-sub karma based on this value. A +100 post on a sub with 1000 members on a technical topic where people tend to downvote for minor errors is worth way more than a +1000 post on a fast-moving sub that's mostly reposted memes.

It is never as simple as "just" doing something. A lot of people have multiple Reddit accounts because it is a forum for everything. Then next is how to determine that arbitrary karma value, what if a lurker has some valuable experience to share? Besides, these are relatively low barriers to entry. Check @weird-eye-issue anecdote below.

You can even buy reddit accounts that are aged(>1 year for like 5 dollars)

You’re only thinking of obvious spam. You need to think bigger to ads that fly under everyone’s radar. You don’t need to burn accounts to do that.