Why the USA didn't just sell Patriot PAC-3 or THAAD to Israel?
Why did the USA finance a different ABM/SAM program?
Why didn't the USA adopt Arrow system?
Politics Stack Exchange is a question and answer site for people interested in governments, policies, and political processes. It only takes a minute to sign up.Sign up to join this community
THAAD wasn't available, Patriot wasn't very effective as a missile interceptor. Not only did you need to get pretty lucky to hit an incoming missile, even if you hit it it was quite likely to cause serious damage on the ground because it wouldn't deflect the missile trajectory enough.
Which isn't surprising as Patriot was not designed as a missile interceptor. It was given some capabilities for the job around the time Arrow development started, which came in handy during Desert Shield/Desert Storm but that conflict also painfully showed its shortcomings. Even multiple launches at a single incoming IRBM would often lead to the warhead and/or pieces of the missile body hitting their intended targets. Something else was needed, and that was Arrow for the Israeli. The US further developed Patriot and the current variants are supposedly better up to the job. And of course they built THAAD and SM-3ER (for the Navy) (originally SM-2ER was used in the function but it suffered from the same shortcomings as did early Patriot models).
So the answer is simply that THAAD did not exist and Patriot didn't match Israeli (and tbh NATO) requirements at the time. Israel of course did buy Patriot as a stop gap and for long range air defense against aircraft.
In fact the first really effective Patriot version for the ABM role didn't become available until around the same time Arrow was deployed (and proved to be better at the job than Patriot).
While the Patriot Missile system was fully built and operational at the start of the Israeli Arrow Missile program, it had a spotty reputation. This was most notoriously on display after the Dhahran failure. Given the imperfect record of the Patriot Missile, a new solution was sought; enter Arrow.
Israel, being on of the US's closest allies, was commonly used as an active testing ground for military equipment. The Arrow missile program was no different. The US financed a large chunk of the program, and Israel was able to test it in real life scenarios. I don't have any citations for this, but my understanding is that the Arrow program was ultimately more successful than the Patriot program given that it was built with more modern technology.
As for THAAD, that program only went live in 2008, where as Arrow went live in 2000, so for obviously reasons THAAD wasn't a candidate.