Because it would end up being more expensive than not consolidating.
First, an important definition: consolidating just means lumping together; federal loans can’t be refinanced to lower rates.
Lumping loans together makes paying them down more expensive because the resulting blob loan takes on the weighted average of my interest rates — currently 7.9% on $63,000, 6.9% on $64,000, and 3.3% on $2,000 — removing my option of focusing on the higher interest rate loans first:
I’d prefer to pay like this:
With a $1,000 monthly payment, consolidating would add up to 11 months and $11,000 in interest to my debt payoff.
My source: This Department of Education page.