I don't believe this is true in the US. Airmail (for "letters" and "small packets up to 2kg") exists all over the world, by international agreement. You can airmail a 2kg package from Mongolia, most probably.
Some countries support up to 4kg; possibly more. IIRC, Sweden used to. I used to be able to tell you but Royal Mail don't seem to publish an easy "book" anymore; it's probably online.
So all the US firms which tell you they can't do it
- are just being lazy
- pay $100 to Fedex/etc under some block deal and charge you $150 (they could not add $50 to airmail)
- are selling diamonds
- are in a market where there is a high level of buyer fraud, so they need continuous tracking (tracked airmail is "recorded" only at the ends of the journey, broadly speaking)
- have customers who are too stupid to understand "shipping time 5-10 days"
- are just being lazy
- are selling to countries where almost everything gets stolen
It is certainly true that in some countries anything which can get stolen does get stolen e.g. Egypt has a theft rate on airmail packages exceeding 50%.
It is also true that 1st World postal services seem to be poorly managed and this creates opportunities for the couriers to rip everybody off.