I understand that the money is debited from an "income account." In the case of income, the "income account" belongs to the company that pays me. It's not _my_ account---why would I keep track of it in my ledger?
it's not than all that's impossible with single entry, it's just way messier, so the overhead of double entry is considered better because the books can be more fully audited.
if you're a freelancer, you may get paid net 30 or 60 or even 90. but you still need to know how much money you have right now, how much money you're owed; financial health, P&Ls etc.