If you have a residential 120/240 single phase service with a perfectly balanced load of 100 amps on each hot leg of a 200 amp service, why is this considered only 100 amps and not 200 amps of load. The distribution transformer has to produce 100 amps of load on x1 and x3 bushings so why do you not add amps.

You add wattage , not amps, of each leg.