Because the technology to get in space is exactly the same as it was in 1967 - primarily liquid fuel rockets with the occasional use of solid fuel boosters. Strictly Newton conservation of momentum is the only thing that gets you off the planet. Gravitational assist PLUS NEWTON is what gets you further out than Mars. Cassini a ONE WAY Saturn probe cost $5,000,000,000 in today's dollars You come up with a quantitative measure of how much it will cost - not some BS you concoct out of air and wishfull thinking. and remember you will need to get sufficient propellant to brake at the asteroid and to break out of solar orbit and return to earth.
The problem with space enthusiasts is that they NEVER figure out how much something will cost - THey read too much science fiction and confuse it with facts
Running around saying it is "too expensive therefore can never be done" is exactly like saying Usain Bolt can't be as fast as he is because you can't sprint that fast with a beer keg strapped to your back.
Running around saying it is "too expensive therefore can never be done" is exactly like saying Usain Bolt can't be as fast as he is because you can't sprint that fast with a beer keg strapped to your back.