Why are the taxes on international tickets so much more than on domestic tickets?

In a nutshell the answer is fuel surcharges and more governments involved! Let’s look at two examples:

Dulles to Los Angeles roundtrip Sep 15-21:
Base fare $342.32
Security Fee $5.00
U.S. Tax $25.68
Passenger Facility Charges $9.00
Federal Segment Fees $7.80
Total $389.80
(taxes/fees added about 12%)
Dulles to London roundtrip Sep 15-21:
Base fare $926.00
Security Fee $2.50
U.S. Arrival/Departure Tax $34.40
Passenger Facility Charges $4.50
APHIS Inspection Fee $5.00
U.S. Customs User Fee $5.50
U.S. Immigration User Fee $7.00
British Taxes and Fees $165.10
Airline Fuel Surcharge $458.00
U.S. Immigration User Fee $7.00
Total $1608.00
(taxes/fees added about 42%)