The first time around you sold a GPU for $5 and took payment as 5 startup shares at $1 apiece. Now you go back for round 2 and the startup says their shares are trading at $5. Will you still loan them the money and exchange 1 GPU for one share at $5?

You get less and less of their company each time and you've already peaked because future funding rounds can't keep being valued higher.

The trick only works once but everyone thinks it's a perpetual motion machine.