If you don't have the failed rudder on hand, you have to build from scratch. However, that isn't as hard as it might seem. Outline drawings are available on-line for pretty much every production sailboat. Those drawings give you the basic shape and dimensions of the rudder. Since you have the boat on hand you also know the rudder post diameter, which constrains the maximum thickness of the rudder. With that information is is a simple matter to choose an appropriate NACA foil for the rudder. Once you have your rudder design you make one up from wood and use it to make a mold. You then lay up the two halves of the rudder. The next step is to make the rudder post. The big question is was the post solid or made from tube. Small diameter posts are often solid, but larger rudder posts are normally made from seamless heavy wall tube. For a spade rudder you will have a decrease in rudder thickness as you go from the top to the bottom of the rudder. That will generally require that the tube be made to taper, which involves some cutting, pressing and welding. The only critical part of shaft fabrication is the welding. Howevr, a competent TIG (or maybe MIG) welder can do the job (by competent I mean someone who knows how to and consistently does full penetration welds and uses the right filler rods for the metal being welded). The next step is to weld the armature to the rudder post. Again you need a competent TIG or MIG welder. The final step is to assemble the rudder. You do that by putting the post/armature into one half of the mold, usually set in resin putty. Once the putty kicks you can fill voids in that half with foam or simply glass the two halves of the rudder around the post/armature and then inject foam into the voids after the glass cures. The potential failure points come if the glassing of the rudder halves together isn't done well (i.e., the rudder delaminates under load and falls apart) or if the post/armature are poorly welded and the welds fail. If your rudder is being made up in a primitive shop that isn't used to fabricating stainless steel and doesn't have a qualified welder, this is likely to be a failure point due to improper materials being used to make up the post/armature and/or poor welding practice. In the worst cast you might end up with a post that is too weak and corrosion prone with a mild steel armature stick welded to it (i.e., basically brazed). If/when the rudder shell leaks, that armature and its welds will rapidly corrode leading to rudder failure. A shop that mostly welds mild steel, might make do with materials on hand rather than order in specialty materials.

Another constraint is that making a quality new rudder from scratch is a relatively time consuming and hence, expensive, process. Many yacht owners might not want to wait 3-4 weeks for a new rudder. Few shops making a one-off rudder are going to eat any of the costs, so the on-off rudder is going to cost a fair penny.

I agree that there are rudder specialists, but I think that is likely due to the need to make (or have on hand) patterns and molds rather than the intrisnic difficulty of rudder fabrication. Molds are expensive to make.

Finally, just because the rudder was made in a small village somewhere doesn't mean that the locals don't have the skills to do the job right. For example, The small towns (less than 3,000 population in two towns) I live and work in have several shops that can and do make rudders at world standards