A lot of what's already said is why, but other things to consider:
Getting to the moon and getting to anything beyond that, is not just the "next step". It's a huge step. The difference is like the Greeks making a boat to get to Italy, and making a boat to get to Hawaii. They both seem the same. Make something that floats and move people. In reality, at the time of the Greeks that could never have been done.
Second, we have made huge advancements, just not in ways that look huge. Take the F-15 for example. The cool thing about it is how it looks, and how it looks when it fly's. The todays F-15 looks about the same when doing that. But the F-15 of today, has nothing in common with the F-15 that flew in 1972 aside from airframe.
You could take 1 modern F-15, and shoot down 10 F-15's from 1972. All this is due to the advancements we have made in aerospace.
