There is a lot involved in bringing new technologies to market.
So let's imagine that Reed Richards sets out to cure cancer. That's a noble goal. What kind of cancer, though? There's over 200 different kinds, and while they all share similarities, they're all different from each other and require different treatments. So, let's say he picks one variety and develops a wonder drug that cures it. That'll be AWESOME for the world...in about 10 years. If he can devote enough money towards all the tasks that need to be completed for FDA approval (I'm not an expert, but it takes many, many trials on animals before any human trials, and human trials are long and expensive).
Taking that into account, how often do you think Reed gets distracted for a day (or week, or month) by needing to go save the city/world/universe/random child? What happens when this pops up and interferes with important deadlines and/or meetings needed to bring his cancer cure to market? You can be certain that intelligent foes (such as Doom) who wanted to oppose him would appropriately time their plots.
Or he could develop the cure(s) for cancer(s) and then give the completed cures to established drug companies, right? Well (tinfoil hat time) that might not go over so well with the drug companies - they'll make a lot less money in the future if they can't sell cancer treatments because there are cures. So those drugs might be found to have 'problems' that require more research before they can be submitted to the FDA. In the real world this isn't very likely (since drug companies would realize that they'd make bank off of the cure, as well as getting lots of good publicity), but in 616 Marvel, where there are conspiracies everywhere and supervillainy is a viable career choice it seems a lot more likely.
So miracle drugs aren't really feasible. But that's fine, let's look at the other stuff we can fix!
I know, how about we create a new technology for powering cars, so they don't rely on fossil fuels?
This is much simpler than getting a new drug to market, since there's no regulatory body that will require huge amounts of testing and research. In fact, if you can produce a car that meets federal safety standards (which are fairly lax, and Tony or Reed could easily do) but doesn't need fuel, you can just start selling it.
This, of course, will meet with resistance from the existing energy industry. Again, in Marvel 616, this will likely take the form of supervillainous attacks and/or conspiracies. There's also likely to be push-back from other economic forces: existing manufacturers will greatly resent these no-doubt-popular vehicles torpedoing the sales of their expensive new models. These manufacturers have inroads into legislative bodies, which can pass laws and/or regulations that will 'incidentally' make your new cars illegal.
A similar argument can be made for any other wide-reaching technology, from agriculturally enhancing areas (you mean, destroying fragile desert ecosystems and deflating food prices? Why are you sending millions of American farmers to the poorhouse, Tony?) to providing cheap, renewable energy to the US and other, developing nations (you mean putting coal miners out of work and giving FUSION technology to TERRORIST COUNTRIES?).
For any development that can significantly help large numbers of people, you can be sure there's already one or more groups economically or politically vested in keeping the status quo. In the Marvel universe, this resistance will almost certainly take the form of a maniac in spandex taking pot shots at you.
Also, there's a personality angle to consider: every super-intelligent person in Marvel tends to have dozens or hundreds of ideas for new technologies. They have an irresistable urge to be constantly working on newer, better things. But spreading a technology to the masses takes time and consistent, constant effort. It's like the difference between sprinting and marathon running. Tony and Reed (and others, like Pym) are sprinters - they work explosively and make great strides in a relatively short time. But they'd make terrible marathon runners.