There have been plenty of extremely well organised, sophisticated and technically competent terrorists. The IRA, the Red Amy Faction, the PLO and Black September, Al Qaeda.
Aside from 9/11 none of them have operated successfully in the US and your domestic terrorism has mainly been perpetrated by individuals or very small one off groups of just 2 or 3.
It's worth noting the Weather Underground were well organized and sophisticated terrorists, although they were admittedly a lot less violent and long-lasting than the listed groups.
There was actually a fair amount of domestic terrorism in the 60’s and 70’s that doesn’t get widely discussed for some reason. FALN was another fairly successful domestic terror group that has gone largely forgotten.
Most people who are idiots are not terrorists either.
On average a terrorist would be smarter than the average person. Terrorists are highly motiviated individuals, they can deeply understand an ideology and they activity learn/train in specialized skills. Compared to the average person their IQ would be 20 points higher.
Being a terrorist isn't easy. It's not a job for everyone.
Looking at what motivated most of the Capitol insurrectionists does not support this claim. They are seemingly unable to distinguish reality from obvious propaganda or wild conspiracy theories.
Most terrorists are idiots because most people who are not idiots are not terrorists.