Why isnt she enough for him!?

I have a 23yr old sis, who is married to a 30yr old man. He was divorced twice, and he was her 1st bf ever. Theyve been married for 6yrs, but she keeps venting to me about how he still misses his 1st ex wife. And havent let go. He keeps talking about and comparing her to his ex wife, especially 1st ex wife. He also said his 1st ex wife is the best he had. She told me hes had this issue w his 2nd ex wife too. Man, idk, she just wont leave him. She keeps saying she needs to get financially stable and independent 1st, before leaving. They have kids together. But he has none w both of his ex wife.

I also dont understand why she isnt enough for him at all. Like come on! Shes 7yrs younger than you, shes the youngest youve ever been with, shes so pretty too, very patient, and forgiving. Id say shes the most beautiful person ive ever known. But hes such an ungrateful asshole. That i always pray and wish for my sister to be able to leave him. I have a feeling she never will.