There are a lot of full-sized keyboards there aka 100%, for example look at Keychron, there are 39 100% and 30 96% along with some variants.

But the obsession with <80% keyboards are steaming from a couple of factors and less soldering as other commenters said is one of them. Other one is what "the community" as many others were overwhelmed at some point by "the influencers" who adored that "digital nomad" lifestyle[1] and ~65% is surely more convenient than 100% one. Another point though I'm not sure it's conscious one is what a lot of people not even using 100% in their life anymore because their devices has a not 100% keyboard from the start - laptops, or don't have a keyboard at all - smartphones. So there is no attachment or "spatial memory" of quickly punching some number on keypad or using navigational block naturally. And at last but not least if you have 100+ keys it's hard to justify all that layers shenanigans and whatnot, heh.

As N=1 I can tell what despite what I ended with K3 Pro (it's 75%) accidentally I'm pretty happy with it because it's compact enough what I can toss it in the laptop bag (though rarely) with my 14" or postman-like[3] with an 8" Android tablet but I still do have the function keys (I was aiming at 65% at first), don't need another keyboard at the desk and 65% isn't that much smaller than 75% despite the looks.

EDIT: oh and don't forget what 90% of the market is just [stupid] fashion anyway, you can't be a serious about MK, DIY and all that when you can buy "solid mega gaming mechanical keyboard with cool anime girl themed keycaps and 95 modes of RGB backlight! (only 3 keycaps btw, but it's not mentioned)" for less than $50 not only from no-name (or RJELTHKETG) Chinese shop but from the big names too.

[0] https://www.keychron.com/collections/all-keyboards

[1] from the home to Starbucks and back making a lot of photos and selfies in process, if you understand what I mean

[2] https://www.keychron.com/collections/keychron-k-pro-series-k...

[3] albeit it's thinner and vertical https://www.victorinox.com/en/Products/Travel-Gear/Backpacks...