It really depends on the implementation. It's not uncommon to see frame locks for example, with cut outs on the lock bar that are noticeably thinner than some liner lock folders. And while lock strength is one of the important factors in a lock, it's certainly not the only factor to consider. Convenience, ease of disengagement, impact on ergonomics are some other things that seem to play a role in many peoples' decisions. If lock strength were the only consideration (not saying that's what you meant), everyone would gravitate towards the Tri-Ad lock. But there are many other relatively weaker locks that can be preferable to users for various reasons. In those cases, the lock strengths are sufficient for the user's purpose, even if there are other stronger lock choices that could be made. And even if it were true that liner locks are the "weakest" (I'm not sure if that's true as a general statement), a well made one can be sufficient for the the needs and wants of the user, which can make the strength question moot since it provides the desired functionality.