Several times I've ordered what I thought was an identical product to something I already own, only to find the fit is slightly different. I'm not sure whether this is due to manufacturing inconsistencies or the style being updated, but it's so frustrating.
I would like to be able to know that I am (say) t-shirt size L and style 'xyz' and body length '3' (or whatever information is needed) and know that I could keep on ordering different products with that same spec forever, and they'd all fit the same. No more wondering whether 'slim fit' means the same thing as it did last year or whatever.
Same with shoes. I've bought running shoes in the same size from the same manufacturer and they've been a totally different fit.
If a company did this and the clothes were actually nice as well, I'd probably be a customer for life.
I'm not a standard size, and it's hard to find clothing that fits well. I hate to go shopping, so I usually wear clothing thin until I'm finally forced to go buy something. And then it's a huge hassle going into shops, trying stuff on, nothing fits... So yeah, given that I already have to waste time to buy clothes, I'd rather waste time getting measured once in a blue moon, and then get well-fitting clothes (presumably with a reasonable choice of colors and styles) based on those measurements.
I do buy tailored suits. They're made-to-measure. That's good enough. Somewhat between true bespoke, the baste is adjusted to fit in-person during second fitting. Good fit for baste is key to a good fit for a blazer. The feel/effect is essentially that I'm wearing the most comfortable T-shirt I have, but it is not a t-shirt, but a suit jacket. Trousers don't really need a second fitting.
For fabric, I really like to first feel and see. Super 100s are usually fine; there's a lot of inflation in higher thread counts.
Mass-produced can do production of good quality clothes without issue, they've done so for decades; for example Armani charge a huge amount for what's a nice fabric but adjusting on final fitting really reduces the point of paying more. Good cloth is worth it, but doesn't need to break the bank. Brands usually don't make the cloth themselves, it is a commodity, if fiddly to get exactly what you want.
One big advantage mass-produced has on custom is quality of stitching. Mass-produced means the stitching quality can be really tight/close and of excellent consistency (take a look at your shirt, how consistent is the stitching? how loose is it? Distance between stitches, quality of thread?). For mass-produced, stitching is certainly not done manually. It is fully automated.
Tailored for wool often works better if the workmanship is top-notch (heavier fabrics) but for cotton mass or semi-mass produced shirts mass-produced often brings a much better stitched that can be adjusted to fit, compared to purely tailored/manually machined/bespoke.
Clarification: I live somewhere where there's a great tradition in tailoring, and prices are not high.
For instance, women's tops are usually sized with measurements for bust, waist, and sometimes hip. However, a whole lot of them don't fit me because I have unusual arm proportions. And it's impossible to know which will because there are no measurements provided for shoulder/yoke, neck, length, or sleeve length.
Without being able to customize all those measurements, a garment doesn't count as "custom-tailored" in my book.
I don't think it would be that much of a hassle, simply have all your measurements taken and keep that on file somewhere. Whenever you go to buy new clothes submit that info and get your nicely fitted t-shirts.
Costs about $10 an item but they fit much better.
Sure, I'd do that in a heartbeat. It'd be great.
If I have to go somewhere and get all measured up, meh: I can usually guess between M/L/XL based on the manufacturer and reviews and for a lot of clothes, that's enough.