The problem is that paradoxically, thicker shields can cause higher doses of radiation due to secondary radiation production. Theres a balance there to minimizing primary and secondary dosage but you cant really get it below a certain threshold without some form of active shielding. Theres a lot of promising ideas for active shielding but they all require alot of power. All this to say that someone on a trip to mars will indeed get a high dose and this is something that needs to be given thought. Its not an intractable problem but it is also not even close to being solved or trivial.

Folks on Mars is still quite a ways away. Not impossible or pointless by any means but it will require significant advancements in many domains before we can really do it safely.