I'm thinking it's because the Sponsor would like to use a car they built from top to bottom. Generally speaking, sponsorship gets you stuff cheap or at cost, not free. While we don't know the economics of the deal, none of us really know how good the deal may or may not be.
What we do know is that a chassis I built is up for sale for less than you could build or buy it for, so someone needs to buy this thing and get it going.
If I didn't already have a 68 A body in the stable, I'd buy this thing back from Tom just so I could beat up their next car with it
Its a well sorted, well built car with a fresh paint job and Tom has driven it brilliantly all year to a much deserved Championship. I have no doubt this car has many more Championships in its future, should the next owner desire.