The raw OSM map data is free (and I am one of the thousands of volunteers that help improve the raw map data all the time).
However, maps based on OSM data are not all the same, or offer the same level of detail, features, routing capabilities, etc. (and are often not in the right format for use on GARMIN devices).
There are many people offering maps based on the raw map data from OSM, but the features, detail included and functionality offered are widely different between each person creating them, as is the level of support offered by these other OSM map creators. Many offer little or no support at all (so you are on your own if you get stuck or have issues).
There are many maps available that use data from the OSM project for their raw map data; however the resulting maps can be very different as the person compiling them can choose exactly what data to show and how to show it. Thinking of it in a different way, let's use the analogy of a meal being made from the same raw material/ingredients from exactly the same source/supplier, but the final meal/dish being created by different types and levels of cooks (from someone that can't boil an egg or warm baked beans up and burns toast, through to a good amateur cook or a professional chef). As you can imagine the results can be extremely varied, from uneatable to sublime!
I have spent over 17 years improving the TT HD Topo/TT 1:50K Look and Feel maps with a custom style and exposing data that is useful to hikers, mountaineers, mountain bikers, greenlaners, ultra athletes, horse riders, geocachers, military personnel, search and rescue and mountain rescue, expidition leaders, mounatin leaders, etc.
My maps are often cited as the most detailed OSM based maps available for Garmin devices.
They also include 10m/33ft contours as standard, and many are also now available with DEM data.
Not forgetting that I offer a very high level of customer support for all my maps, including the FREE ones (now Donation, raising funds for the Mountain Rescue Teams across the British Isles) I offer on my site...
All my TT HD Topo maps are updated at least once a month; the British Isles map is updated every week. The FREE (now Donation) maps are usually updated about every six months.
As you can imagine this is a huge amount of work and to be able to deliver the high level of customer service and constant product updates that I do, I have given up my full-time day job in computer security. This is because the maps now take up all my time.
Every month I create over 440 new (updated) maps for almost every country/region/state/province/federal district across the globe!
GARMIN also uses the raw OSM map data now, and they also charge for maps based on the same data, even though their OSM based TopoActive maps are not as detailed as mine, often not even having contours set at less than 50m in them (Europe). The GARMIN TopoActive maps are also not updated very often (only about once or twice a year).
I often get comments that I should charge more for the maps (most of my customers), or charge less for them or not charge for them at all (a small minority). This just goes to show that you can't please everyone.
To those that say the maps should be free, I say that's fine, but I still have to pay the bills, and if you think creating and maintaining maps and multiple websites, and supporting them is trivial, I say go ahead and create you own maps then, and offer them for free and you will soon see just how non-trivial and time intensive doing this really is...