For whatever reasons, being that obtaining a license from these companies is a bit steep cost wise or they perhaps may not want such a product made bearing their name. Who knows!
having seen "one of those letters", I know. There are phrases like 'no replication of the logo', no 'visual modeling of the likeness', 'no representation of any mark or corporate identity', 'no identification of or with the products of...' -some then discuss licensing fees, others are flat refusals or 'cease and desist' threats. Most certainly some manufacturers are benign toward the Flight Sim world and others are even generous and helpful, but as 'small-shop' modelers, who wants/dares to test the legal departments of large corporations looking for billable hours?
BTW, the corp. world is funny/sad too... I've also seen the 'cease and desist' letters go out to guys posting photos of corp. airplanes on the web (won't hold water, although there is a legal opinion that it might if people were visible embarking/disembarking and could be identified)
As well, there's a market issue. Granted some of the tubeliner guys would buy (but some only see careers as Airline Pilots!, not corp. taxi drivers), some of the warbird and Golden Age guys might spring the buck but perhaps a smaller buck than for the latest, greatest piston-popper -or they wait for the one particular aircraft out of a hundred so which one wins?
Granted, those of us who love
anything with wings are more amenable, but remember that the FS community can be pretty fractured so the market for a "less-exciting" (not as big as an Airbus, to clean for a Wasp lover, too genteel for the warbird guys) is likely going to be smaller relative to the development time for a high-quality model (and we keep hearing that all models must be near perfect :isadizzy

.
We'll see some of them as time goes on, but look at FS9.. there are still huge gaps in the catalog after 7 years. Today, the factories are often still grinding out the 5th Mustang, third Corsair, 20th tube to get a share of the bigger markets that are well-known. You know what happens if a shop builds something a bit 'different'.. there are immediately hundreds of post saying "nice, but I wanted a.... and I have a budget"
BTW, I'm holding out for an early Sabreliner and/or an AeroCommander/Rockwell 1123)

AND I'm on a budget...
Rob