There is no explanation why there's an inconsistency between regions where tms break and don't. Here's a joke, what does the Pokemon even do to learn the move? Does it eat it like a berry in the earlier games? The more obvious reason is because of either limitations or an added sense of challenge and strategy. Even so, I prefer multi use or a tm shop in a department store or special npc which sells tms you used up, so you can purchase them again if needed. Glitches such as Missingno, PC duplication in GBC and emerald are ways to duplicate items including tms, circumventing the situation. Albeit, missingno is harder for me to use. Gen 4, even though my favorite gen, has to be the hardest gen to farm tms and technically get enough to feel like multi use tms because there isn't as many glitches or exploits in the originals besides multiple play throughs then trading them over for more powerful/useful tm moves. BDSP requires to be on a outdated version to perform it's infamous duplication glitches easily. Gen 9 just wants you to use the material gathering gimmick, which requires some farming, but at least you can get all tms over again with recipes.