For 3D printing stuff, you obviously need a 3D printer, these aren't free, the maintenance isn't free, the space where you are going to put it isn't free. There are also the basic tools: screwdrivers, wrenches, pliers, drill, soldering iron, etc... Also missing are consumables like glue, solder, grease, cleaning agents, etc... and also electricity and gas for shopping. You are also generally expected to have a proper computer.
Sometimes jellybean parts like screws, wires, resistors, etc... are priced, but it is often not the case, as it is difficult to price 2 screws when you can't buy less than 100.
Not the case here but the worst is when a part list include significant parts you are supposed to be salvaged from something you already have. No, I don't have the right broken appliance in my garage and no one has conveniently dropped one in the closest dumpster. So please price it as if I had to buy it.
Of course, someone who is into building stuff most likely have a lot of that already, or maybe is part of a makerspace, but it is unfair to compare what represents a significant investment in time and money to just buying a complete product on Amazon with free shipping and a warranty.
Warranty is another thing to consider, because when something goes wrong with an off the shelf product, if you bought it through a reputable seller (Amazon qualifies), you can normally return it free of charge. If you screw up on your DIY build, which is very common (failed print, bad cut, magic smoke, lose small parts, ...), you get to fix it on your own, savvy people buy spares for that reason, again, not priced in.
For everyone else, I would be very surprised if they didn't have most of those things on hand or would gladly use this type of project as an excuse to buy more tools. That's how DIYers build up their tool chests, a series of excuses. But guess what, these are not one time things. I bought by 3d printer with an excuse and print tons of stuff. It's just how the hobby goes. Hell, I've torn apart a bunch of old electronics from Goodwill when I was teaching myself how to solder (highly recommend this btw), including how to microsolder. Why? For fun? I definitely don't get paid for it (though at one point I sometimes did).
And yes, people like to build and tinker as a hobby.
The thing with building stuff is that it feels like you are always missing something. Like, you have all sizes of screws, including some that look like they come from an alien spaceship, but not the one you need, so here goes another box of 100 for the 10 of them you will use in your lifetime.
> an excuse to buy more tools
It used to be the case for me. Not anymore for the simple reason that I live in an appartement and don't have a dedicated workshop and I feel like I am drowning in parts and tools. The worst part is that I actually may need all of them, but I don't know which one. It turns out that I finally used the turbo-encabulator that has been sitting on shelf for 15 years, and of course, I had to buy new marzlevane grease, as I didn't want to risk side fumbling.
And yes, I have that hobby, and I too like to pretend that I built that thing for dirt cheap, but the reality is that it cost me a lot more than I would have paid otherwise, that's the cost of having a hobby.
3D printing could be an exception though. I have printed plenty of small parts for essentially nothing (except time) that are almost impossible to find at a reasonable price. Also great for gifts, as you can make objects that are personalized and useful, enough to be appreciated, but cheaply and with limited (but non-zero) effort. I hesitated a long time before getting a 3D printer, because it is an investment and I couldn't think of many use cases at the time that didn't involve adding more clutter to my clutter, but it turned out I probably use it more than anything else.
The part about a lack of space is serious though, it has always been a frustration, more than time and expenses. Not enough to warrant major changes in my life though, it is just a hobby among others.