Sure, such things should be a natural thing for such creatures to do. But RAW they aren't--and that's the flaw.
An Ogre doing grapple things IS RAW. How is it not? Where does it say monsters can only do things in their star block and not do general purpose attacks?
By my saying "such things
should be a natural thing"--I mean a GOOD thing. By that I mean something they can do and expect a reasonable chance of success. Ogres trying to grapple most characters, even in tier 1, are lucky if they will succeed half of the time. Let's think about this:
Ogre gets +4 to a STR check vs. a PC's STR (Athletics) or DEX (Acrobatics). The Ogre only wins if they roll higher and loses on a tie. IME even a Wizard or other caster will typically have a DEX +2. At the higher end, you have +4 or +5 even, and a maximum with expertise of +7 is possible.
Even at the low end of +2, the Ogre's chance of success is 57.25%. Ok, but not great.
At the higher end, say +5, the chance drops to 42.75%. Depending on the target's AC the Ogre might be better off attacking (where it
will win ties).
At the extreme end, say +7, the Ogre's only wins 34%--barely 1 in 3 attempts.
Next, what is the benefit of grappling? Not much really. The grappled creature's speed is 0 and you can move up to half your speed carrying/dragging the creature. Meanwhile, the grappled creature can still attack normally and cast spells, and isn't taking any damage from the Ogre. Meanwhile, if the Ogre attacks, it will hit about 50/50 and any hit should drop most level 1 PCs (avg. dmg 13).
Now, IMO grappling is not a good option for Ogres against most PCs, even at lower levels. So, as I said, RAW they aren't [good at things they
should be naturally at]--and that's the flaw."
As to the rest of your post. Nothing says you can't do things in the stat blocks, and I never claimed otherwise. I already acknowledged that you
can attempt such things. I just don't think it generally is a good idea because it is not as effective as attacking.
I can't speak for others, but for myself, the issue is that many creatures can try to do such things, but often times those efforts are not very effective. I see this as a design flaw myself, because many such actions should have a good chance of working--but just don't. For instance, Ogres have STR 19 but no proficiency in Athletics. A level 1 PC with STR 16 and proficiency in Athletics is +5 versus the Ogre's +4. So, the ogre would more likely fail in the attempt than succeed.
So, to put an end to it: I
DO think Ogres should be better at grappling and other STR-based moves, but if you look at the numbers they usually aren't, so it isn't an effective strategy.
That is why I say it is a design flaw.