There are very few campsites except on the Costa del Norte
I guess if you don't look for them you don't notice them, but that is not completely true.
There are far less campsites than for example in France and Germany, but there are quite a few at least on / close to the Francés (I did not do any research for other routes). On the Francés there are over 20 official campsites on/near the Camino route
(or at least within a more or less reasonable detour, depending how much you prefer to camp)
Campsites directly on or very close to the Camino Francés are for example in
- St. Jean Pied de Port,
- Camping Urrobi after Roncesvalles is only a short detour
- Puente La Reina (the albergue up the hill is also a campsite)
- Estella (a bit before town)
- Ayegui (camping Iratxe just after wine fountain )
- Castrojeriz
- Carrion de los Condes (down next to the river and park)
- Burgos (on the alternative river route into the city)
- Sarria
- Portomarin (maybe 1,5 km outside of town, but has a very good restaurant)
- Arzúa (really good pool!)
- San Marcos before Santiago
- Santiago
and more. Those are just a few that came directly to mind. Plus the albergues that allow to camp.
No idea about the via de la plata, though.
So, sorry for going off-topic (because my information is for the wrong Camino route and the op seems to ask about wild camping more than official campsites) but that blanket statement is simply not correct and I couldn't resist.
Since the title of the opening post does not indicate which route, those searching the forum for info regarding camping on the Francés might read this, too, so I hope it is okay to leave this comment. If not, feel free to remove it of course.