Because he literally said so, and has a history of not being particularly fond of her in general and not actually apologising for saying he hated her from what I saw and read and it doesn't read like satire.
But I was giving you the benefit of the doubt to prove it was satire that simply went over the line since I don't actually follow him and I may have missed any statements regarding that.
Here he is confirming it was a lazy Games of Thrones reference.
Clearly he's saying it was supposed to be a joke. A form of satire that he's been known for almost his whole career. Do you honestly believe Clarkson thinks Megan is a worse human being than Rose West?! Come on.
Do you think Jo Brand was being serious when she suggested people should throw acid at Nigel Farage? No. Of course not. It was shocking humour.
If you can't see this as satire then satire truly is dead.