I think a couple things are in play:

• Very few people consumed these comics as we are... reading each one in sequence. You'd more likely sporadically encounter them in the funnies section of a physical newspaper. Which was a pretty hit/miss proposition to begin with. No one expected every one to be a winner, and people would routinely skip over stuff that didn't interest them without thinking about it too hard. You're operating under the assumption that Far Side is a classic, but at the time people would just cruise by and think "that comic is stupid, just like 60% of the other stupid comics on this page". And folks were pretty happy to have 40% of comics be a bit funny.
• What made Far Side a classic was not its consistency. Rather, there were a few strips that became cultural phenomena. Basically a handful of hits that were breakout memes of the 80s and 90s. Colleges used to sell t-shirts of the school for the gifted strip with the kid pushing on the door that says pull, which is pretty accessible and one of those breakout hits.
• Because of those breakout hit strips, some folks got into Larson's style of humor enough that fewer of his strips were inscrutable to them and he had a lasting market.
• Other comments point about topical references and those are also a big deal. If someone sees a beans meme with no context 30y from now, it ain't gonna be funny. But a few weeks ago on lemmy, it was part of a contextual zeitgeist that was more or less about "these idiots will upvote anything, I'm one of the idiots... I'll upvote this!" and it kind of captured the exuberant excitement of not knowing what lemmy was but wanting it to be something. Similarly, these strips often weren't intended to last multiple generations. They assumed you were reading the newspaper RIGHT NOW... and so could reference current events very obliquely and still be accessible.

TLDR: Like a stupid meme, many Larson comics require shared transient context we're missing now. Some are also just fukin weird, like cow tools. But some were very accessible and became hugely popular. These mega-star strips cemented Far Side's popularity, and which gave Larson the autonomy to stay weird when he chose. Now we waste time trying to figure out what they meant.