It's probably just me but I find your question confusing. First I'll try and answer as asked.

I would have to say most people don't think about financing the growth of their future retirement income because they don't know that is an option. How many ads do you see offering a loan on investment account growth? Not to mention the fact that I doubt regulators would look favorably on an advisor recommending to take on debt for the purpose of investing. Not to mention I can't see a lender offering funds for that purpose at any kind of reasonable rate.

Now I'll take a shot at answering what I think you are actually asking.

Folks don't think twice about getting a loan for a major purchase like a home or car and obligating a large chunk of their monthly take home income. So why won't they allocate a similarly large amount to something every bit as important and necessary as retirement income? The American population is woefully illiterate when it comes to financial issues. Retirement is one of those far off things that has been given no urgency by our society in general. Retirement savings is not viewed as giving any immediate perceivable benefit. Therefore why would people allocate part of their income to fulfill an unrecognized need that they won't benefit from?