I think President Trump rightfully sees that Iran is the biggest threat in the area. IMHO he is building a coalition to counter Iran and this arms deal is part of it. Also aircraft cost a lot of money. One more thing is Saudi Arabia does not have to buy these arms from us. They can buy them from China or Russia and Europe.
And at least four ships.
The Saudis desperately want to buy from the US. They want to be linked to the US and have the US as their ally. In 1991 when Saddam rolled into Kuwait, the Saudis knew they faced an existential threat. If Saddam has listened to some of his generals, he would have called himself a “liberator” and would have seized all of Arabia before the US concentrated its forces within two months of the Kuwait invasion. The vile, corrupt 9/11 Saudis know that the Russians , Chinese and the Europeans without the US would never have come to save their royal behinds. If not for the US they would have been dust and know that their survival depends on their American “ally” to be the guarantor of their regime. Of course they would turn on the US in an instant if circumstances changed.