While some of it may have been hurried lawmaking or carelessness, I think the lack of means testing was simply that it was administratively much easier to only include an income test. While we debate the means testing issue here in the whole scheme of things the number of those of substantial means receiving subsidies are the proverbial angel on a pinhead so it wasn't worth the effort and complexity of including a means test and chance losing a crucial vote because of the added complexity it created.
On the second point, while people with higher incomes do get higher payments under SS because they paid in more, because of bend points, those who pay in less get higher benefits in relation to what they paid in and people who paid in more get less in relation to what they paid in... so SS benefits are definitely skewed to favor the less fortunate at the detriment of the more fortunate... to content otherwise is just ignorant of how the bend points work.