You really answer your own question, "who in their right mind would pay for this": obviously the value is competitive to warrant the higher price. I don't know the respective airlines' loads on this route, but it's a fair bet that Emirates is holding its own.

Some reasons to choose Emirates even though it costs more, and accepting arguendo that the on-board service is the same on all three:

-- Skywards membership
-- Slightly better frequencies (a check shows that EY has 2 flights from LON and 3 to BKK per day; QR has 4 and 3; EK has 5 [from LHR] and 5) (not including code-shares)
-- Better aircraft (A380 on EK)
-- Better airport in which to connect

Of course are free to not value Emirates and choose an alternative. But obviously not everyone agrees with you.