Why is Saudi Arabia buying so much military hardware?

Because Iran is.

because they can, and they can afford it.

what else are they Going to do with the Trillions they make on Oil

Pretty much to protect itself and to spend money on the U.S. as a political favor (so the U.S. can keep spending even more money on defending it).

Saudi Arabia is not very well liked in the Middle East, because of its dealing with the U.S.; so it needs the hardware to defend itself in case the U.S. decided not to bail them out. Currently, Saudi is at war with the Houthi rebels in Yemen as well as supporting the U.S. and the allies in defeating ISIS (on paper anyway).

to protect themselves from Iran and other Islamic terrorists.

They're getting it while they still have American dollars to spend. Sooner or later, their oil is going to run out, and then they won't be able to buy anything.

because there are a lot of other muslim countries that think they should be in charge of Mecca

Iran for one

Building up to fight Shiites over who gets to control the caliphate before Iran develops their nuclear capabilities.

Saudi Arabia is having a lot of internal problems as more people are unhappy with the Saud family monarchy running the place. They are also enemies of Iran who appears to be growing steadily stronger. They are trying to keep control both inside and outside the country. They are also strengthening ties with China.

They may be afraid Trump will make a deal with Russia for us to take all the arab oil then stabilize the world price which Russia sorely needs. Or not. You actually think anyone swimming in this cesspool will have a rational answer to your question? Most only know what the media tells them!