Problems getting harder is not really a valid explanation for the current state of science, or any particular field. At any given glimpse of society, their current state of the art is the best they've managed with all they know. Further progress is always harder, even when we now think of it as something that should have been easy. For instance the basic concept that things happen for a reason is something everybody understands now, even only a completely intuitive level - yet it was one of the breakthroughs of the Greeks. And Newtonian mechanics is also something people tend to understand quite intuitively, yet following the Greeks it would take the better part of two thousand years for people to consider that e.g. objects in motion stay in motion unless acted on by something else. Artistotlean mechanics held that continued motion required continued force. Artistotle seemingly did not consider friction, air resistance, and so on that cause objects to slow. Or just consider the tens to hundreds of thousands of years to develop writing, or to enter into the iron age, and so on.

There's some argument to be said that in times past there were fewer "scientists" working on these topics, explaining the delays. But I don't think that's fair. The reason I put "scientist" in quotes is because many of these things have minimal prerequisites and the average home would have all that's needed to experimentally test and discover these concepts. And their immense value, far beyond the academic, meant their discovery likely would have been able to rapidly spread regardless of the origin - so that, at least to some degree, precludes authority as a necessity.

Many things that now constantly elude our ability to understand, perhaps dark matter is a great example, will likely one day be child's play.