In short, the market is seen as too small and not willing to pay the prices those companies feel they need to charge to justify entering it.
Think of it this way - if Atmasphere were to have made say the Balancing Act, they would charge something like 8-10K minimum for it - but how many users are willing to pay that much? 10? 15? It would be a low number, not one that would justify the R&D and production costs on a new line.
This may all change - as more flagships are introduced and speaker users are gradually innoculated with the notion of headphones being more than just an "accessory," the high end headphone market will gain numbers and have more members with deep pockets who still see headphone setup prices as a bargain (and/or a necessity in a quiet house at night or an urban environment.) At that point I wouldn't be surprised to see respected speaker amp companies joining the fray. They'd still have a hard time of it as the one man operations can undercut their prices, but at that point (the logic goes) there would be many speaker users familiar with the brand name who would prefer to buy their products and would be willing to pay the premium.
Personally I'm happy with where the market is right now - the pricing insanity that has infected a large portion of the speaker world has largely passed over the world of headphone amps, with the possible exception of one or two of them. The headphone world is competing for who can do more for less, whereas more and more the speaker market seems to be about who can charge more and the cachet pricier products bring. Obviously this is a generalization with exceptions, but I feel it to be true overall.