My guess for Archeops is that it was some sort of balancing decision. I can't speak on competitive, but in-game in its debut generation, Archeops is really really good even with the worst ability ever. Maybe the idea was to make it the most glass canon ever? It's clearly meant to be one with those stats, so I could see them wanting to take the archetype even further by making it not need to lose all of its health to be effectively off the field.
Whatever the reason, it can be guaranteed that they weren't thinking too hard about comp when they made it.
Well it definitely was a pokemon designed with it in mind. "This pokemon will have good stats, but a bad ability"
But the question is still "Why Archeops?". Like it's not really part of the theme of the criter its based on or any lore, as said before, and its partner in crime Carracosta is a fairly normal pokemon with no equivalent gimmick. Like Scrafty says they could have just had a normal rock/flying pokemon, with lower but still okay stats and a normal (perhaps dull) ability.
With Slaking you can see the logic: it's a sloth, so it'd be funny if it had an ability where it lazes around. And since it's skipping an entire turn, let's go nuts and give it 670 BST.
With Regigigas they wanted to go with the "sealed away, moss growing on it for sitting around so long" aesthetic so it gets Slow Start, then said let's go nuts & give it 670 BST so it feels real powerful when it gets going.
Archeops is just...an archeopterix. There's nothing about its behavior that really lends itself to have a similar, but lesser, gimmick like this.
At the very least I feel like a better ability name, one that could used for both, would be something like "Exhaustion" with the conceit being that now they're tired from trying to fly at all (for archen) or all the time (for archeops) due to all the damage they've taken.
It'd still be weird to make Archeops a Pokemon with this kind of gimmick, but at least there'd be some proper flavor.
While I'm back seat game designing, if I insisted on making Archeops like this I'd probably give Carracosta a similar ability with a similar quasi-busted-but-not-really statline. And lore to match; make it the timid one or something idk.