It was harder for Columbus to reach the west and bring back treasures than it is for us to reach the asteroid belt and bring back exotic materials. What is missing is not technology (need is the mother of invention) but a reason to go. Was Columbus 100% sure his mission would be profitable?
What if Columbus waited another 100 years for a safer option to cross the mysterious expanse? History would be much different.
Attitudes toward risk are a lot different now as well. No one would bat an eye at ships being lost at sea, or a ship returning with a quarter of her crew. It was considered inherent that any chance at exploration or glory would come with mortal danger.
Our current attitudes towards exploration is more like the large corporation. Playing to avoid loss, rather than playing to win.
What is missing is not technology but a reason to go.
"At 1997 prices, a relatively small metallic asteroid with a diameter of 1.6 km (0.99 mi) contains more than 20 trillion US dollars worth of industrial and precious metals." [1]
Spot-checking (pun not intended), those metals are currently up as much as an order of magnitude over their 1997 prices. So that same small-ish asteroid could be worth something more along the lines of a quarter petadollar.
If that's not sufficient reason to strip mine the asteroid belt, I don't know what is.
I bet the value of metals on Earth already far exceeds this many times over. And what do you think would happen to the price of metals if you had this huge supply?
Is this really true? Getting to the moon cost 4% of the federal budget at peak, and from the graphs it looks like about 2% of the budget on average, for 13 years [1]. I'm fairly certain that Columbus' voyages cost nowhere near a percentage point of the economic output of all of Europe for a decade. But maybe I'm wrong. Could you provide a citation?
Also, as I understand it we're fairly certain of the economic value of other planets given our current technology (not much). So that aspect of Columbus' voyage does not apply today.
Columbus needed massive funding (got it from Queen of Spain IRRC).He didn't need a reason, he needed to convince some moneybags the risk was worth it. US Politics is incapable of taking any risks.
US Politics has considerable few dictatorial monarchs who can literally silence any opposition, don't have to answer to the general public and don't have to care if the general public is starving to death.
What if Columbus waited another 100 years for a safer option to cross the mysterious expanse? History would be much different.