As an example, read American Airlines Conditions of Carriage, specifically Carry on Baggage and Liability.
The suitability of carry-on baggage will be determined by American. Each customer will be limited to one carry-on bag and one personal item. A personal item includes a purse, briefcase, laptop, or small book bag style backpack. A personal item must be smaller than your carry-on bag and must fit under the seat in front of you. Fragile or valuable items, such as keys, medication or computers should be carried in your personal item.
That's the first point. A personal item wouldn't be checked, since it is supposed to fit under the seat, not in the overhead, and all valuable items should be in it, and not carry-on baggage.
The second point is under liability:
American does not assume liability for any of the following items in or as checked baggage: antiques, artifacts, artwork, books and documents, china, computers and other electronic equipment, computer software, fragile items (including child/infant restraint devices such as strollers and car seats), eyeglasses, prescription sunglasses, non-prescription sunglasses and all other eyewear and eye/vision devices whether lenses are glass, plastic, or some other material, furs, heirlooms, keys, liquids, medicines, money, orthotics, surgical supports, perishable items, photographic, video and optical equipment, precious metals, stones or jewelry, securities and negotiable papers, silverware, samples, unique or irreplaceable items or any other similar valuable items.
If your carry-on is checked, and it contains valuables, they are not liable. Period. Valuables belong in your personal item.
Next paragraph starts out:
With the exception of strollers, American does not accept these items in or as checked baggage and assumes no responsibility or liability for such items, regardless of whether American knew or should have known of the presence of such items in checked or transferred baggage.
And then, the piece de la resistance is paragraph 6 under liability:
American assumes no liability for articles carried in the passenger cabin.
It is my assumption, that every other airline, large or small, will have quite similar statements as these in their fine print.
The bottom line: mail, and insure.