I just realised that lenses are basically pieces of glass with focal length matched to a particular camera. I'm wondering why we cannot use any lens from Canon, Nikon , etc if we use a suitable adapter ring? Is there such a product?
There are adapter rings for different brand of cameras, however there are restrictions. One is the distance from mount flange to film, where this distance for the lens must be equal or larger than that for the body mount, otherwise there will be no infinity focusing.
For example Canon EOS is 44mm while Olympus 4/3 is 38.67mm. Many current mounts have larger distances so can be adapted to these 2 with infinity focus. Pentax K and M42 have the same distances of 45.46mm.
There may be mechanical obstructions. Canon EOS can't fit into Pentax K completely and no infinity focus due to flange distance. Pentax K lens have a protrusion which may interfere with mirror of 35mm / full frame bodies.
Depends on which mounts you are looking at. It is possible to use Nikon, Leica R, Contax, M42 lens on Canon EOS body with stop down metering while most non-rangefinder lens can be adapated to Olympus 4/3. Pentax K works fine on Canon APS bodies with smaller mirrors, this one I tried myself.