I don't think this is true. Part of optimisation is finding synergies. And the whole point of a synergy is that a new strength emerges from existing elements that are worth having on their own account.Optimizers always weaken something to get that good. If the DM isnt capitilizing on that at least occasionally then sure they plow through everything.
I don't think that generalisations about RPGs per se are helpful here. Because different RPGs can be very different.RPGs aren’t like 10-pin bowling or like basketball. In both those game, there is one clear objective and clear rules about how to obtain it.
In RPGs, devoting resources to crushing each combat can, depending on the table, be completely beside the point.
In Classic Traveller there is not really such a thing as optimisation: almost all of PC build is random, and one of the few points of choice is to allocate generic combat skills to particular weapons. There is scope for optimisation here - because different weapons have stat ratings below which a penalty is incurred and above which a bonus accrues - but it's not very complicated.
Prince Valiant is another RPG in which there is no scope for optimisation. PC build is just choosing skill ranks. It's not a system that has a number of interacting mechanical parts. There are no synergies.
D&D stands out, to me, as a RPG which has almost absurdly many interacting PC build elements and the same number of interacting subsystems. This is what creates the scope for optimisation; and also what creates the scope for building sub-optimal PCs if a player picks a build element that on its face seems to speak to what the player wants his/her PC to do, but which doesn't actually generate that pay-off in play because of the way it interacts with other elements of build and resolution framework.