It's filling the form in, taking it to the vet & handing it in, getting the cheque, going to the bank to pay the cheque in etc etc . Just plain laziness
I'm pretty lazy too but the promise of cash usually shakes me out of it I keep the forms in the car so I always have one with me when I go to the vet and I fill it in then and there as I'm paying and they do the rest.
Like Jan, the hassle of taking the form back to the vet, collecting it again and then sending it off it too much hassle for us for small amounts - we have a £35 excess, but now that the standard consultation fee is £25+VAT, I rarely get charged under the excess, but don't bother to claim unless its over £50 or so
Our vet won't fill in the form there and then - it is done by the administrators, so I have to go back and pick it up a few days later.........
We got a letter from the Practice not long after we got Molo saying that the FSA guidelines meant that vets should no longer send the forms to the Insurance Company direct, or accept payment direct
Once the cost of petrol, and my or OH hourly rate is taken into account for all the to'ing and fro'ing it doesn't make it worth it for small amounts