I don't agree with this. After all, most players like to level their PCs and the effect of levelling a PC is that you have to fight 4 kobolds rather than 1 (or a bugbear instead of a kobold, or whatever).
I can't say how things work at your table, but I've never seen a random encounter chart where one of the entries was "a level-appropriate number of kobolds."
What you are describing is an instance of the infamous treadmill, by which advancement is rendered meaningless. It's
only meaningless in the face of meta-gaming by the DM, whereby enemies change relative to the power of the PCs, though. If the DM is roleplaying their NPCs appropriately, without taking into account information that those NPCs don't possess, then
most groups of enemies should remain consistent in power
regardless of how competent the PCs are. If you're stronger or more experienced, then you will have an easier time at killing that kobold (or whatever other monster is in that room), which is exactly as it
should be in order to make sense within the world. (Some enemies may change, depending on
their foreknowledge of the PCs and their abilities, but it would require an informed enemy who knows that this party in particular will be their opposition.)
The benefit of being more competent, or higher level, or whatever is (i) you get the game play satisfaction of having worked the system well, and (ii) you get the story-type satisfaction of, in the fiction, being someone who beat four kobolds. Some players enjoy (i) more than others; I suspect many D&D players enjoy it at least a bit given that D&D is a fairly mechanics-heavy system. I assume that most RPGers enjoy (ii) at least a bit, because it is one of the main reasons to play a RPG rather than a board game or wargame.
You could not be more incorrect if you were trying. Which you might be, for all I know. It's hard to discern tone over the internet.
As for (i), the goal of optimization - if you're into that sort of thing - is to increase the likelihood of success. If players wanted to optimize, by showing off their system mastery, then they would have to take into consideration that their opposition would scale relative to their own abilities. If that's how the game works, then they would have to account for it if they wanted to "win" that game. It would be like playing an older edition (pre-4E), where the DM promises to not include traps if you don't have a thief/rogue, and to increase the natural healing rate if nobody in the party is a cleric; suddenly, a party full of fighters becomes the optimal solution, since they will have fewer obstacles to overcome. If gaining levels
causally means you face harder challenges, then you should stop gaining levels at 6, because anything further puts you into sudden death territory (speaking in 3E terms); or to use a video game example, Final Fantasy VIII is significantly easier if you stay at level 1 the whole time, because enemies scale to your level and
they get more out of each level than
you do.
More importantly for an RPG, though, if you did that then you would be putting the player into conflict with their own character. Optimization
is how you roleplay a competent character who wants to survive. If my fighter character chooses to bring a cleric along, and invest time in learning advanced techniques with the greatsword, then it's because
they expect that will increase their odds of successfully doing what they need to do; if the player knows that those choices are meaningless - or even counter-productive - due to DM shenanigans, then the best choice for the character is no longer the best choice for the player.
As for (ii), it is simply not true that a story about overcoming greater opposition is necessarily more satisfying than a story about overcoming something more down-to-earth. It might be more satisfying
if you didn't know that the enemy was specifically tailored to oppose you, but that will never be the case as long as the DM is contriving encounters based on your own ability. I could write a story right now, about how the Magnificent pemerton single-handedly fought off three dragons and then went on to raise Atlantis, and it wouldn't mean anything. Stories are inherently meaningless, unless there's
something which gives them that meaning. In an RPG, that meaning comes from the fact that the GM
isn't contriving things for you, such that the outcome of your actions really
are your own.