Reloading for a belted magnum is still expensive because the front of the belt is a sharp angle with the case body and is a real stress riser. Cases often crack at that point after only a few firings because it is in the thinner part of the case, unlike a rim which is well behind the thickest part of the case.
The original reason for the belt was to provide case support (headspace) for cartridges like the .375 H&H Magnum. That cartridge was designed to be rimless, but it has almost no shoulder. Without a rim or a shoulder could easily be driven too far into the chamber even with a manually operated bolt action, and also could cushion the firing pin blow for lack of case support. So the belt was a compromise to get the feeding of a rimless case and almost the support of a rimmed case.
The problem came about when folks assumed that the belt was to strengthen the case head and began to equate a belted cartridge with power. So cartridge designers put belts on cartridges for pure hype, thus (inadvertently?) making sure shooters would have to buy a lot of nice new cartridges or at least brass if they wanted to shoot very much.