Saudi and Iran have long fought each other through proxy wars throughout the Middle East. However when Iran started its nuclear program, how come Saudi didn’t do it in response?
Firstly, it'd be a violation of the Non-Proliferation Treaty [NPT] in Article II and other articles, as it's forbidden for non-nuclear-weapon state parties to research, proliferate or manufacture such weapons.
Article II [NPT]:
Each non-nuclear-weapon State Party to the Treaty undertakes not to receive the transfer from any transferor whatsoever of nuclear weapons or other nuclear explosive devices or of control over such weapons or explosive devices directly, or indirectly; not to manufacture or otherwise acquire nuclear weapons or other nuclear explosive devices; and not to seek or receive any assistance in the manufacture of nuclear weapons or other nuclear explosive devices.
However many ratifiers of this treaty, Iran, North Korea, which withdrew from the NPT make other nations seek such weapons as they fear for their sovereignty which makes this treaty not always applicable as nations may be forced to violate such treaty. Even the US many argue with NATO broke this treaty, when the US gave nuclear arms to NATO Members, which would violate Article I of the NPT but NATO argued that such actions were to stabilize a treat met by Soviet Union
- Saudi Arabia is in a difficult position, as it doesn't have many nuclear expenses or materials to pursue such a weapon. With even their economy based on petrol and oil having no thoughts on nuclear energy, there not even any nuclear reactors.
Because Saudi Arabia lacks any nuclear reactors or meaningful quantities of nuclear materials...
- Saudi Arabia could get nuclear arms from Non-Signatories of the NPT such as Pakistan, but this would sour the relationship with Israel and the US and the US might place sanctions and even stop weapons sales with SA. The USA is somewhat an important partner to handle threats met by Iran
In February 2019, a U.S. Congressional report indicated that Trump administration officials had pursued a nuclear reactor construction deal with Saudi Arabia. The deal attracted controversy for allegedly bypassing the 123 Agreement process stipulated by the Atomic Energy Act, which requires Congressional approval for sensitive transfer of nuclear technologies to non-U.S. countries.
But there are indications that SA has an interest and might enter manufacturing nuclear arms and developing nuclear technology. Prince Mohammed Bin Salman has made some remarks
There are many reasons, at first, Saudi Arabia doesn’t need to have strong army to counter Iran, it’s clear that if there is any conflict in the Middle East, other powers and above all the US will decide what will be the result of the war, as it was the case in Iran-Iraq war and Iraq invasion of Kuwait.
Besides, Saudi Arabia has signed NPT and any waiver from this treaty will have very bad consequences for Arabia, unlike Iran, Saudi Arabia doesn't want to face sanctions. Iran has nothing to lose, its economy has collapsed and has chosen to fight the West and bear all these consequences, Iran has the weakest currency in the world and its economic growth in 2020 was -7.2%(rank 189 out of 192 countries) and the same thing will happen to Arabia's economy if it is sanctioned.
In addition Saudi Arabia has chosen other strategies to reduce the threat, such as working with Israel. The US-led Warsaw conference in 2019 was a conference that was destined to bring countries of the region to counter threats, caused by Iran and after that conference Arab countries decided to reduce tensions with Israel above all to face Iran threat.
So Saudi Arabia has chosen a rational way to counter belligerent behavior of its neighbor without breaching international laws and bringing about arms race in the region.