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Generally photographers put UV filters on DSLRs for the larger purpose of protecting their lenses. Lenses can cost hundreds or thousands of dollars, whereas UV filters are generally less than $40, and sometimes less than $10. 

UV filters are practically useless (image-wise) on DSLRs since digital sensors are not nearly as sensitive to UV light as film. So don't spend $150 on a "premium UV filter" if you're shooting digital. Just grab a $20 one off amazon and use it as a lens protector. 

Compared to the body, certain types of lenses can be extremely hard to make. Quality aside, if you want a 1.4 aperture 300m telephoto lens, that will be extremely expensive to make (the resulting lens will be huge with lots of glass and components, and most likely impossible to build practically).

Some digital cameras can adjust for lens focal problems. Sometimes the focus was sharp enough but other times, especially at f/2.8 (limited depth of field) the shots were soft. This was a film camera and no local calibration was possible. 

The lens manufacturer starts with a mixing of chemicals and fusing them in high temperature, making the glass. The glass is fused, coooled, cutted and polished, and you have a lens.