This does not quite follow. Pretty much any ecosystem is more complicated than any of us can understand, yet scientists are able to make reasonable predictions of the effect of a single factor even if they can't describe the end state.
We never know the exact positions of atoms in any system yet we are pretty good at knowing the macro behaviours of solids and solutions.
We cannot know the precise state of any chaotic system but we manage to make pretty accurate prediction of the weather, an incredibly chaotic one.

Just because something is too complicated does not mean we can't say anything. And, to be fair, quite a few economists (those who did take real-world evidence into account) did have useful things to say. A major problem came from the fact that people in positions of power were able to buy the type of advice that they wanted.

Many economists realise that -I was at a conference at LSE and asked a few questions to the speaker on this line, and the answer was that people in power will probably not change the way they view the world, but he was hoping that his work would have some influence over next generations of decision-makers.

One the one hand I fear he is deluded -there will always be enough economists ready to be bought to sprout wealth-serving nonsense. On the other hand, IMF and OECD have finally changed their line somewhat. Belatedly and imperfectly indeed, but recognisably so. So maybe they do have a bit of very long term influence.
Although it is yet to translate into political action, so whatever influence they might have is depressingly slow, but not just because economists want to retreat and hide. Krugman and Wren-Lewis have been quite vocal in expressing their frustration that policy was in total contradiction to what economics suggested.

Earth provides enough to satisfy every man's need, but not every man's greed. Gandhi

The world economy is more complicated than any of us can understand and getting more so by the day.

And this is stated every time an economist talks about the economy in media or to politicians.

Let us first tackle the question of the nihilistic component of fundamental uncertainty. As already pointed out, it must be admitted that those authors who have emphasized the importance of uncertainty have generally overestimated its destructive consequences for economic analysis. Two destructive paths have been pursued: one underlying the presumed irrationality of the agents; and the other the instability of the economic capitalist system. The reader should be convinced by now that ontological and epistemological uncertainty does not of necessity breed irrational behaviour. In fact, can be argued on the contrary that, both in the Treatise on Probability and The General Theory, Keynes is striving to define a realistic and practical theory of procedural rationality based on the limitations of human knowledge and capabilities. When deprived of knowledge, reason cannot be based on simple probabilities and must turn to alternative strategies based on conventions and other procedures. In this context 'Keynes may be viewedas basing the whole of economic theory on a single, broad, non-Neoclassical conception of agent rationality' (O'Donnell, 1989, p.272)

The second path towards nihilistic conclusions follows some of Keynes' arguments. It has been assumed that uncertainty leads to instability since long-term decisions depend on flimsy foundations, subject to sudden changes (1973, xiv, p.114). This has meant to some that a proper theory set in historical time, where these violent changes in opinions would have to be recorded, is beyond the reach of economics. Sticking to Keynes for the moment, it is well known that he also considered uncertainty to be a stabilizing influence on the economy, since a variety of opinions and the confidence with which they are held ensures mitigated aggregate reactions to news (Keynes, 1936, p.172). The position taken here is that the presence of fundamental uncertainty, combined with a rationality based on procedures, generates regular patterns, except in exceptional crises (bifurcations and the like).

...

The implications of all this are that models based on rules of thumb, such as mark-ups, target-return pricing, normal financial ratios, standard rates of utilization, propensities to consume, lexicographic rules and so are perfectly legitimate since they rely on a type of rationality that is appropriate to the usual economic environment. In a world of uncertainty and of limited computational abilities, the economic agent cannot but adopt, except in the simplest of problems, a rationality that is of the procedural type. The models based on rules of thumb are not ad hoc constructions as the mainstream would like them to be because they are not derived from some axiomatic optimizing formalization. Rather, they reflect the rationality of reasonable agents. As such, they have macroeconomic foundations that are more solid, from a realist point of view, than those of the standard mainstream models. Indeed, when one thinks about it, what is more ad hoc: to assume that economic agents follow some rules of thumb and are satisfied once they reach some threshold; or to assume that agents are omniscient, omnipotent and maximize some utility function that contains no conflicting goals, while at the macro level the representative agent acts as both a consumer and a producer? Heterodox economists must feel at ease in dismissing such accusations; it is orthodox theory that makes use of ad hoc methods (Amable et al., 1997). Making use of ecological rationality is perfectly legitimate because this is how people truly behave.