They wanted to have Messi too but the player refused to join Saudi Professional League. They wanted to have him to make a legendary fight between Messi and Ronaldo in Saudi Arabia but that plan failed.
Benzema and Neymar are not Messi and Ronaldo but they are one of biggest star players in last many years. Their injuries only show risk of signing contracts with old players even they are star players and professional. With their old ages, it's hard to maintain good body and fitness like when they were younger years ago. Neymar has bad habit in his life so moving to Saudi Arabia will give him more opportunities to enjoy his life with very lucrative salary and special privileges from Ah Hilal.
Obviously salary is the main attraction for moving to Saudi Arabia. Messi rejected the offer mainly because of opposition from his family members. For the European players, it maybe quite difficult to get adjusted to the rules and cultural norms in Saudi Arabia since there are no pubs and bars. On top of that, playing in the Saudi Professional League may not give them the same exposure that they may get while playing in different UEFA leagues. But all of this will go in the background, when Saudi clubs are ready to pay 4x or 5x the current salary for these players.