UA doesn't have BKK flights. If UA is flying into NRT/PEK/SIN/whatever, they're probably timing the flight for a good timing for departure from the US destination and a good arrival time into NRT/PEK/SIN, not "hey, let's optimize the arrival and departure times for our Asian flights for someone who wants to connect to or from BKK on a partner". Thus you will see late arrivals into BKK (for BKK-NRT you're flying a TATL-length flight AFTER a TPAC, so add six-seven hours plus layover time, so no wonder the arrival times blow).

Eva Air (BR) has North American flights that leave from the US at oh-dark-hundred and TPE in the late evening, and are timed pretty well for a BKK daytime arrival and departure, if that is what you are looking for. They're also in *A so you might even get UA miles. Their business class is well-regarded and not impossible to get on miles.