To me, the loaded coins/tokens/objects have an allure about them that the others just don't and typically won't. Also, for the most part, a DIY loses tradability once assembled, really no fun in that. If one is going the route of cold storage and being their own bank for their sats, then DIY makes more sense.
I wont disagree on that aspect of the allure. But really, the tradability is the same - you still have to trust that the person holding it (not the manufacturer) did not tamper with/compromise/replace the private key - while not super easy, it is also not impossible to remove a holo, scan the private key and replace the holo without someone able to tell - can the average person do it? probably not. But it has been done.
Basically, unless you buy from the manufacturer you have no way to know it is still intact and untampered with - you go from trusting the manufacturer to trusting every single person who laid fingers on it.
To each their own - I guess maybe a compromise would be to offer it both ways - the manufacturer loses nothing to sell it as a DIY.
edit:
"Not your keys, not your coins". Stay vigilant, never put more on someone else's keys that you can't afford to lose.
I whole heartedly agree with the above.
Every key I have generated for anyone - I have said the above to - almost verbatim.