Why is there so much prehistoric art found in Spain and France?

Here's a bit of theory, with the next paragraph being a bit more near fact. Pretend that the map you had has some geography in it. Notice that the area of Northern Spain and Southern France is the Pyrenees mountain range. During the time of prehistory , having high ground was prefered because it was a strategy to score easy kills on Woolly mammoths. So I believe that the mountains would provide a sort of "camp" so that they could go near the foot of the mountain to hunt any mammoths that happen to come by.

It is very likely that humans hunted cave lions and bears according to fossil evidence. This means that ancient humans might have liked to go to caves, which are very common at the mountains.

Sources:

hunted cave lions

hunted cave bears

how mammoths were hunted

The first reason is one needs an environment where caves exist. Call this either geology or topology. The larger the caves the better because they can offer greater protection to greater numbers of people (hominids) against weather and predators and they provide easier access.

The nature/geology of the cave walls would also be a factor. Walls whose surface would be permanent or little compromised over time are required to retain artworks on them. Walls that are friable and prone to flaking will either not take a sharp image and they will not keep the artwork on them, as over time the artwork will fall off with constant renewal of the surface of the walls.

The third reason is the region needs to be frequented by people/hominids. Access to pigments helps, but these can be brought in from where ever they were at the time.

Access to food and water would also be a factor as this wold enable the creators of the art to be settled for a period. External climate would be another factor; anything under an ice sheet cannot be accessed. An external climate that is too hot or too cold would be too hostile.

The internal climate within the caves would also be important. The absence of ultraviolet radiation from the Sun would be needed to protect the artwork. Similarly, humidity within the caves needs to be constant within a specific range to prevent artwork from becoming either too moist or too dry and falling off as a result of either condition.

Caves with entrances that are obscured so they are not easily noticed would also help, as this would limit the access to such sites over time and the subsequent potential for damage to the artwork by people just being there over time and touching things.

There is not more prehistoric art in Spain and France.

This is because prehistoric studies were started on France, plus add a language barrier and a huge archeological search bias.

There is plenty of prehistoric art in the Balkans, as well as Morocco, Libya... and other non-developed, poor, second-tier language countries in southern Europe that have no funding for historical research, nor to pay for translations for you to read in English. Italy and Greece have as much prehistoric art as France and Spain, but they get less butget for it because other historical periods are preferred to get tourists atracted.

All these countries have caves. Caves are everywhere, it's not a preservation bias, although certain caves preserve better than others and have a local bias, it's not something exclusive of Spanish and French regions.

Also, it's not a prehistoric cultural thing, there was nothing different in Upper Paleolithic Cantabria compared with say, Romania, both were of Aurignacian culture.

Note that for all prehistoric findings there is a significant selection bias related to where archeologists searched for it. Western Europe has been checked and searched for prehistoric evidence of human settlements considerably more thoroughly than any other region of the world. This effect is slowly getting less and less important as archeologists are searching more and more places on earth in ever greater detail but to a large extend the answer to 'Why are most findings of prehistoric art in Western Europe?' is simply 'Because that is where most people where searching for it.'

The Franco-Cantabrian region appears to have had a series of early cultures that valued art. It has also been continuously inhabited for far longer than most parts of Europe, having apparently been a refuge for humans, animals and plants during the Last Glacial Maximum. There was time for a lot of art to be created, and conditions have been right since then for it to be preserved.

Once it became clear that there were multiple caves with art in the region, checking all known caves for more art would be natural for modern cultures.