Why is it difficult to travel to other planets?

Answer 1

It is much more difficult to send unmanned probes than it is to send manned missions, for obvious reasons. Supporting human life during the years-long trips to other planets and back poses many formidable problems around issues like air, food, water, waste management, protection from radiation, maintenance of mental acuity and mental health, maintenance of bone strength and muscle mass, adequate communications systems, general medical care personnel and materials, enough fuel to move and to stop all that stuff, and many others.

Answer 2

they can't because as of the technology that we have is not good enough and it would take too long and cost way too much