In one factor, Benzema's belief was the thing that made him interested in playing in the Saudi Arabian league. Apart from that, in terms of age and salary, the offer is a second choice for him to play in an easier league due to the aging factor. Also another thing is the existence of the figure of Ronaldo, who is likely for him to be one of the challenges to accept this offer.
Al ittihad are the reigning champions this season and they seem to be continuing to make changes in the squad to increase their strength for next season. With the arrival of kante and Benzema I think they are relatively strong next season even Al nassr will most likely find it difficult to stop Al ittihad in the next season. There are still a lot of players who are likely to join the SPL this season.
I think the money is the only reason and nothing else. I mean this dude won't be making this much in his life, for a few more seasons as player and nothing else after that, this dude will not make this much money ever, he was offered money he wouldn't earn ever again.
That is why he didn't turn it down, there aren't that many players that would be worth 100 million euro as a transfer fee let alone as salary, famously the biggest one is 60 million and mbappe if I am not wrong, with the loyalty bonus (signing fee) that becomes a bit closer to 100 but still not there. This dude is making 100+ million a year easily, that's just unheard of, and he is almost at retirement age as well. I am pretty sure that there aren't that many players who would reject that, only like handful of players who would and they are mostly young (and apparently Messi as well).