Let's directly dig into the content.

OG, the official GMAT Preparation material, has close to 1400 problems and even if you buy all the versions the total # problems without overlap will come close to 2500 problems. Lets just say 500 problems from each section and you have got ten topics in each section so that brings us down to 50 problems for each topic. That is the maximum number of problems available for each topic and most of them are easy only one third of them are difficult, and you can solve close to 70% of the problems easily, so how can you get the maximum benefit from such a limited number of problems. When I was initially preparing for GMAT my review would be to cross check the answer, find what went wrong and then redo it (most problems in mind - Yeah! This was just a simple mistake). You wouldn't care to redo/review the correct ones. It will be like that because mistakes would be careless ones, techniques or concepts. You would not bother about careless mistakes and techniques and worry only about concepts. With very few problems at hand, how can we improve the quality of learning. The only way available is reviewing.

GMAT is all about quality and not quantity, so solving more number of problems won't guarantee us an increase in score. To get better quality we should review effectively.

Quant problem - find the easiest possible method - there is always a simpler way. E.g. Inequalities problems can be solved in algebraic approach or through trial and error. Similar manner for some geometry problems. Search through forums, get familiarized with different approaches and learn the one which gives you highest accuracy in shortest time. Take notes, prepare flash cards and review them daily.

SC problem - identify every error available in every answer choice. Verify the errors, the best way to remember concepts. Search for every error explanation in strategy guides or in forums, look for expert replies [saves lot of time]. Even if there are two to three errors in a single choice you get close to 10 mistakes to learn from a single problem. You miss so many information when you don't review.

CR problem - think of an alternative answer. Most of the GMAT
CR questions imitate real world scenarios - they give a plan and ask us
to find the one which most strengthens/supports/weakens ? You can always
think of such options in real world scenarios. If you have a hard time
doing those - try to find what each answer choice does to the question,
some might be irrelevant, some be doing the opposite, find those and
cross check them, again make use of the forums.

RC problems are quite different in this aspect. While there are ways to review the problem effectively, I haven't followed a concrete approach to review RC problems, it has been easy for me ever since I started following Ron's approach to solve RCs. You can sort of zoom out and zoom in to search for answers and find them easily.

In the end, its all about reviewing effectively, even though you have only close to 50 problems per topic, you get innumerable methods, approaches and ways to learn from a single problem, which is equivalent to solving many problems. And by the way check this link for GMAT Instructor Ron's take on how to review the problems effectively.