What are the main reasons why fast (military) jets can’t fly all the way across large oceans and rely, instead, on being transported as air cargo?

Good question. Let’s take…. this one for example.

A-10 carried to Afghanistan on a C-5.

1. The distance between San Fransisco to Kabul is 11949 kilometres for example or somewhere around 7424.9 miles, that is way too much for A-10’s 1287 km range to go there directly as it requires a lot of refueling in air or in allied base. C-5 on the other hand has a range of 11705 km so if you want to transport it from say from somewhere around California to Kabul then it only requires 1 refueling instead of around 15 times the A-10 requires.
2. Sometimes you only need to transport a few aircraft and to send it the other way (land or naval) requires a massive effort as you need to escort and make a big convoy (land) or you only have big ships (naval) so for small amount of aircraft transported you can use aircrafts.
3. Aircrafts reach the destination faster than ships. With the top speed of 932 km/h, the C-5 is way faster than any cargo ships that the US armed forces have so it is useful for fast transport to replace aircraft that is damaged or requires replacement for other reasons. And if it is transported by ships, the aircrafts still need to be carried by land from the seaport to the airbase while if transported by the aircraft the package can go straight to the airbase.
4. 

   

   

   051003-F-4466M-002 People line up to enter the 445th Airlift Wing’s first C-5A Galaxy after it arrived at Wright-Patterson Air Force Base, Ohio, Oct. 3. The Air Force Reserve Command unit will receive 10 more C-5s to replace its retiring C-141 Starlifter aircraft. (U.S. Air Force photo by Tech. Sgt. Charlie Miller)