maybe they make them disposable because there just aren't any people out there anymore who know how to work on this stuff. Same reason there are PLEK machines-allowing unskilled people to do a luthier's job, while charging more money.
Except that any major city has plenty of people qualified to fix amps. Maybe not 'guitar amp' techs per se, but the guy at the local used hifi sales/service place will do just as well.
I know 1 guy who supports his family (and has for years) with gear repairs (tube, point to point and solid state/pcb stuff). He works out of his house and he's the best guy in town (nice dude too). I think he also takes in all serious repairs from one used shop accross town (that their horrible tech can't handle) and works one day a week as the 12th fret's in house guy. His prices are insanely good (when you got to him direct) because he's lightening fast (just really knows his stuff) so the hours are low. I was very pleasantly surprised at the cost for him to go through my 16:8:2 console, and I always take my tube amps to him.
Then there's the local vintage Hifi guy (almost as good, but more expensive... and a little weird) and at least 4 stores (one a multi-location chain) that will repair stuff, but I have no idea about their quality of work and pricing (except one which I refuse to name or go to anymore since their tech put a Traynor Super Custom Special on the floor with an under-rated fuse, so that when I rented it it died before I hit a note... and then when I called he told me to just go get the right fuse at the hardware store like what he'd done was no biggie - what if I first fired it up at a show, after stores had closed?). Then there's all the pro audio/studio support guys some of whom also take amps and other things (but would likely not be cheap), a guy who designs/builds pedals and is known to do repairs, my recone guy (who also does amps, mostly PA stuff, but is too far away for me to use vs the other local awesome dude) and places like Teletech - I don't know what they'll work on, but I assume more pro audio stuff (too far for me to go there for repair).
That said, some of the disposable stuff might not be worth working on and techs might reject it, but I don't have anything with SMDs (aside from a pair of passive stepped attenuators made by hand by some dude in Asia that I couldn't resist due to price - actually good; SMDs make sense herebut I have no idea how the dude does it - with a loupe and very carefully) but some techs have mentioned that caveat when I brought them things (not about SMD specifically, but just, in general - other problems could be proprietary chips or sealed modules; think Standel's epoxy-potted stuff etc.)
Anyway, the reason is profit, not lack of techs. But it is a bit self-perpetuating maybe in that techs might not work on the stuff.