1. Yes, most of the time they are not brands as such. Lots are independent designers, with varying degrees of experience, some might be college students, some others are seasoned designers in other fields, most learned how to make a board for "fun", and usually this is done in their free time after their regular jobs. With the tight margins that most boards tend to run, paying someone to make a website that's barely going to be visited is not worth the cost, or the time investment that it requires from oneself even when you are hiring someone.
2. The higher priced items are going to be out of stock as they are run on small batches as that what many vendors can afford to do. It's easy to think "stock 1000 boards, they will eventually sell", but where does the initial capital to buy the production of those 1000 boards come from? But there are many vendors that have multiple switches and caps in-stock, as well as boards, many of they fairly budget oriented and a great place to start experimenting.
3. As others said, the process is expensive, specially for small-runs like it's usually done in this hobby. Designers should also be paid for the work they've put working on that board/product, as well as the vendors making that sale possible. Each prototype made is expensive, that cost needs to be compensated in the final price. Aluminum is expensive, the effects of the pandemic on transportation and supply are still very much present in the prices of many materials and components.
4. If this is worth the price is for you to answer. Some designers and vendors have lower margins, others have much higher margins, there's even some that are running without profits. Either case is fair as they value their time and effort differently. Intentionally limiting the number of boards to a low number can be to ensure they can fulfill orders reasonably fast with good QC, other might want to create a more exclusive product by having a smaller round, either case is fair. It's up to you to decide if that sticker price is something you can and want to afford.

> It would be nice to hear how you manage to have fun with the limitations I mentioned above

I like to build through-hole kits, there's usually a few in-stock on different places (or when they are done in a GB model they usually fulfill pretty quickly) and it's a cheap way to play around with different layouts and test switches.

Why are you in the hobby? What is that you want to get from it? For me it's that experimentation, to try different things and how certain mods affect different boards and switches.