THe game doesn't do any such thing. The DM however might. The game does not determine what you will face on a week by week basis where the DM does. A DM can force optimization in D&D, GURPS, Savage Worlds, WoD, or any game they run.
It's not purely on the DM. If one player makes a strong (mechanically) character and another makes a weak one, then any situation which is a good challenge for the weak character is pathetically easy for the strong one, and a situation which challenges the strong character is far beyond the abilities of the weak one.
Nor is this always a question of deliberate optimization. If you've got a party of people who "build to the concept," and one person's concept is "I'm a [3E] wizard who doesn't like to kill people, so uses control and debuff magic instead of direct damage," while another person's concept is "I'm a monk who [the rest of this sentence does not matter]," you're likely to end up with a major power discrepancy, regardless of intentions.
Now, not all groups have a problem with power discrepancies. The monk player may be perfectly happy just doing the monk thing and letting the wizard do the heavy lifting, and the wizard player may be happy doing the heavy lifting. But if not everyone is happy in this situation, you've got a problem which cannot be addressed simply by the DM adjusting the challenge level.