Somebody else may be able to explain it better but the way I see it. Smith, Marx etc were venturing to describe an economic system that was just coming into being at the time (capitalism) and as such spent a lot of time engaging with the history of economic organization. They do a lot of work to arrive at how this system works (like, coming up with the labor theory of value and things like that) but are always conscious that all they’re doing is describing a system which is the result of the collective choices humans have made as a result of a historical (and Marx says dialectical) process with awareness of the fact that we can continue to reorganize society and evolve along that same process. Marx especially worked on developing the theory of how such historical progress might be made.

Economics, on the other hand, the way it’s taught and practiced now in capitalist society, takes this system as basically a law of nature. They talk about supply and demand curves like they’re describing the law of gravity. Some of the fundamental premises that all actors in the system are rational economic actors and that the market will self regulate to equilibrium and all this other stuff are basically false but taken for an almost religious truth. The whole industry of economics is set up to help capitalists make better decisions and predictions within this system using mathematical models with no mind paid toward whether these systems can and should be fundamentally reorganized to better meet the needs of society. They took the politics out of “political economy” to make a pseudo-mathematical discipline which they call economics. They mostly don’t content at all with Marx, they just pretend that his critique never existed. I’ve often thought of modern mainstream economists as fulfilling a role similar to the clergy in the old feudal system, existing to reinforce and justify the current order and the ruling class and propagate the ideology that benefits them. That may not be totally fair, I don’t want to say that economics is useless or that it doesn’t give us some good insight into how public policy can benefit people because obviously it does some of that too…. I’m kind of rambling, hope somebody else can jump in with a better or more concise explanation.