The reason we care so much about inequality has nothing to do with jealousy and everything to do with power[1]. As resources are a major source of power (and often vice-versa), and because power is relative, inequality of resources means inequality of power. And as power means influence (and so the ability to reduce another's freedom), inequality reduces overall freedom.
As freedom can never be total[2], the question is do we prefer to reduce the freedom of the powerful few (whose freedom is already large) in order to increase that of the many, or vice-versa?
So while absolute income is certainly very important, ignoring inequality is simply irrational for someone who values freedom.
[2]: One person can be totally free; two cannot. X would either be free to infringe on Y's freedom or not -- in either case, freedom is no longer total.
As freedom can never be total[2], the question is do we prefer to reduce the freedom of the powerful few (whose freedom is already large) in order to increase that of the many, or vice-versa?
So while absolute income is certainly very important, ignoring inequality is simply irrational for someone who values freedom.
[1]: https://en.wikipedia.org/wiki/Power_(social_and_political)
[2]: One person can be totally free; two cannot. X would either be free to infringe on Y's freedom or not -- in either case, freedom is no longer total.