Here are some reasons.
1. A very important reason is that while the insurance is cheap/reasonable like in the US, for international parcels you can only buy insurance for expensive classes of mail (like Priority Express) that cost about $50 for the postage alone, not the cheap First Class International Parcel.
2. I don't think you can buy international tracking. It seems to come free with the customs number, but only for certain countries can you track with the customs number.
3. You need complicated custom forms over a certain amount.
4. it is complicated. Different countries have different rules as to the amount of insurance they cover, or if they cover insurance at all. Prohibited items that can be sent vary by country. You have to look at the list of countries and examine what the country in question allows.

OK to self-insure for low price items.

Robert
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Rock Hudson (right) and Julie Adams in "The Lawless Breed" (1953).
http://www.antiquegamblingchips.com/StillsD_Westerns.htm