It's not so much that the default rate is higher, since rates can compensate for that, but that the defaulting loans stop paying comparatively sooner..
The graphs below show the probabilities of defaulting loans to stop paying, for each month until maturity. Highest risk is at the 11th month for 36-months loans and 13th month for 60 months loans, so respectively at around 1/3 of maturity for 36-months loans but only 1/5th for 60-months loans. Therefore if a 36-months and a 60-months loss both default, the loss will be greater on the second one.