Actually, the Martian atmosphere makes it extremely problematic from an entry, descent, and landing (EDL) standpoint; the atmosphere is thick enough to pose a real problem with aeroheating, requiring thermal protection systems (TPS) to insulate a payload, but is too thin for subsonic aeroflight or deceleration for a large (>1 metric ton) payload. The payloads that have been delivered to Mars to date use enormous and highly reinforced parachutes deployed at high supersonic speeds but these are highly stressed because of the highly dynamic pressure that comes with the speed and force of deployment. (It is counterintuitive to many that the forces are higher even though the atmosphere is ~1% of the density of Earth, but because of how thin the atmosphere decelerators have to be deployed at very high body speeds.) In the ‘Eighties and early ‘Nineties a lot of work was done on ‘bluff body’ biconic descent capsule designs but they stop becoming effective for lift at supersonic speeds, and more recently work is done on inflatable conical decelerators (e.g. the Low Density Supersonic Decelerator program) but even they have limits of how much payload they can land, and for crewed mission a descent capsule is going to be >6 metric ton even if you separate the bulk of supplies and equipment from the crewed vehicle. Supersonic retropropulsion is effectively the only way to land a large capsule but that requires carrying a large mass of propellant al the way to Mars, substantially increasing logistical requirements and complexity.
Other issues are weeks long dust storms which essentially prohibit reliance upon ground-based solar power (necessitating nuclear fission as at least a backup and supplementary power source), the erosive texture of and toxic perchlorates in the Martian regolith, lack of any radiation shielding in the form of a thick atmosphere (against cosmic rays) or magnetosphere (against charged particles) which will require shielding (most likely by digging several meters under the regolith), and the general immature state of in-situ resource utilization (ISRU); while a proof of concept of extract oxygen from the Martian atmosphere by the Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE), the ability to do so at scale within a realistic energy budget is still in question. As noted above, the only ‘liquid’ water to be found on the Martian surface is in the thick brines of recurring slope lineae in which the water is bound so tightly it doesn’t evaporate in the near-vacuum ambient pressure of the Mars atmosphere. It can be extracted by heating but that begs the question of how much energy that will take and how to deal with the large waste of those brines (again, including a lot of perchlorate salts) that will remain. There are likely subsurface frozen ‘lakes’ at the poles but the ice there will be so cold as to be as hard as granite and still likely contaminated with salts. It might actually just be more feasible to collect ice-bearing small asteroids and send them to the Martian surface rather than trying to extract or synthesize water in-situ.
There remains that challenge of astronauts experiencing freefall conditions for months and then having to adapt to even the ~1/3 gee of Martian gravity, and even how much that lower field will do in ameliorate the various physiological issues of not experiencing Earth-normal gravity; not just musculoskeletal degeneration but a whole host of problems that occur in freefall conditions and which space physiologists will also be a concern in low gravity fields.
It’s not just a matter of space being ‘frictionless’; there is additional impulse required to achieve escape from Earth’s sphere of influence (SOI), injection into orbit around Mars, and descent to the surface. This is only ~23% more in velocity (delta-V) versus landing on the Moon’s surface which doesn’t sound to bad but the reality is that there will be so much more mass of equipment, propellants and other consumables, shielding, et cetera for a crewed mission to Mars versus the Moon that it is several orders of magnitude more effort even for a very minimal mission profile, much less a conjunction-class mission with a ~560 day stay at Mars and all of the equipment that would be required for building any kind of long term base with shielding and recycling of consumables. All of that additional mass might just seem like ‘logistics’, but for a complex and technically challenging mission like this changes in scale are significant challenges in the difficult, cost, and reliability of the mission.
Stranger