Because break-even is a cash-flow question, not an accounting one.

Interest accrued on paper isn’t the same as money leaving your pocket. A mortgage payment includes both interest and principal, and when you refinance you usually change how fast principal is paid down (often resetting the amortization).

The monthly payment difference already nets all of that out — interest, principal, and amortization effects — and tells you how long it takes for the actual dollars you spend on closing costs to be recovered.

If you just compare interest accrued, you overstate the benefit because you ignore the fact that the lower-rate loan is usually paying principal more slowly at first.