There may be no grand objects to be found, but that does not mean there isn't much to find. The thing that we will find is more knowledge about those societies.
We already know relatively little about Neolithic Europe despite there being major cities there that rivaled or eclipsed those in Mesopotamia in population. Yet we hardly know anything about them in comparison because they used perishable materials for almost everything (and it’s not only because of writing). Finding anything useful underwater makes that many times harder.
What are you referring to? I could certainly imagine such a thing but I can't say I've ever heard of someone finding evidence of a city that matches this description.
e.g. Maidanetske site could have had 20-40k inhabitants. Uruk possibly only reached that population centuries later.
But of course accurate estimates of this and other similar sites are very tricky.
Submerged places... imagine how complex and slow archaeologic excavations look on the ground. Now move it 50m underwater in endless mud. Good luck looking for literal needle in the muddy haystack of 40,000km2. To find what... some fossilized wood or few arrow heads? That culture was very similar to ones on neighboring lands, no atlantis there.
One day, robots can scan and dig through there but there are so many better ways to spending money, any money, that it makes perfect sense nobody is doing it.