Why did Frodo feel he had to leave Middle Earth at the of ROTK?

[Inziladun]

Originally Posted by Lotrelf

I have a question. Why does professor use the word "Arda unmarred" for the Undying Lands while it was marred by Morgoth and Ungoliant?

Although Ungoliant did kill the Trees, the land of Aman itself was not harmed, corrupted, or affected by Melkor, as he had done in Middle-earth. Aman was the design of the Valar indeed "unmarred" from that viewpoint.