They sell software and if hardware requirements are kept at 15 year old levels just because a small portion of customers still use literally ancient hardware, it may be a bad business decision if it means that it makes the game unattractive for new customers.
15 years.
15 years ago 3D accelerators were in their first generation. The most popular was the add in board 3DFX, which came, IIRC, with 4mb of video memory. Have fun trying to run AH on that.
When EverQuest was released in 1999 it required a 3D accelerator, the first major game to do so, and it was seen as a hugely risky move on Verrant/SOE's part to require that. EQ's graphics compared to live AH are terrible. The detail shown in that preview is better than World of Warcraft's 2004 release graphics and has higher texture resolutions than anything in WoW did until the end of the Wrath of the Lich King expansion when Blizzard boosted the resolution on player armor sets. Of course a fantasy MMO like EQ or WoW only needs to display a few hundred yards of terrain at a time whereas a flight sim has to display miles of it, and the more miles it displays at once the better it feels.
In the end AH's (nor IL-2: Battle of Stalingrad's, nor War Thunder's) graphics are ever going to match a concurrent first person shooter's graphics or a third person MMO's detail level. They can't and still provide usable view distances and frame rates. The plane, and tank, graphics are the only places they are likely to compete as those are seen up close. AH's plane graphics range from good on the newer units to terrible on the B-26B which dates from v1.00 released in 2000. HTC is obviously not unaware of this as the newer units are obviously built of more polygons. HiTech recently mentioned how many they use, but I don't recall the number, but it has gone ever up through the years. Compare the B-26 from AH v1.00 with the A-20G from the end period of AH1 with the Ki-84 of AH v2.00 with the Mosquito Mk VI from 2010 with something recent like the Yak-3. There are clear improvements at every step.