Model-specific endcaps are always the superior solution. If they exist for a given hub, you know they will work. Universal adapters that do this threaten to be gimmick products. There's nothing that says a given 12mm fork will have a flat surface on the dropouts sufficient to pair with a 15mm hub. In a lot of cases it may be fine, but it's not a given. You've also got a spare thing in play during wheel changes. The exact length of the tubular sleeve can't be any longer than the hub OLD or else it can be dangerous, and any shorter and it can rattle. There's just not a lot of good reasons to do it that way, since in the best case it's another precision machined part to buy and all it's doing is adapting to something that's not any cheaper or more useful than getting the right hub/wheel in the first place.
Some of what you're asking gets into design elements of front hubs, and at that point the conversation is more about tradeoffs than generalities. Many 12x100 hubs can have their endcaps switched to 15x100 or QR, but not all. In general the ones that can go all the way up to 15 will be stiffer and heavier while the dedicated 12mm ones will be lighter, because the "main axle" component of the hub internals can be smaller. How much that stiffness contributes meaningfully to anything is probably pretty negligible. All else equal, dedicated implies more latitude to spec bearings that are whatever you want for your durability vs weight needs, whereas having a hub that's capable of fitting a 15mm axle and instead is wasting space endcapping down to 12mm implies you could have bigger bearings instead, or less mass somewhere. In reality it's not much of a functional concern, but it is there in the design considerations. It's unequivocal that the absolute most efficient designs are dedicated.
Though as you mention there are some geographical factors, 12x100 front hubs are common at this point. It is the main standard in use for new road bikes, and every hub and wheelset manufacturer makes them. If you're not seeing them, it's possible you're looking somewhere that's very mountain-focused.
What there isn't necessarily a large variety of to choose from is aftermarket 12x100 hubs that are readily available as hub only and below the premium price points. In other words, it's easy to buy the high end, but getting something low- to mid-price can be a little niche. Shimano often does a good job being the biggest source in that market, i.e. with the HB-RS470 or R7000 hubs in this case. (To be clear, all Shimano hubs are dedicated axle, no endcaps).
