Saudi Arabia is an "old friend". Yet all the 9/11 hijackers had Saudi connections. What an odd coincidence.
The Saudi Royal family benefits enough from its relationship with the USA to the point that they would never directly sponsor anti-American terrorists. However, the way they stay in power in their country is by paying off fanatical Islamist clerics. In return for largesse, the clerics tell the faithful to remain loyal to the House of Saud, and then they use the money to sponsor jihadis and fanatical interpretations of Islam abroad that are actively anti-western.
That's why it's no coincidence that Al Quaeda grew out of Saudi Arabia.