Tesla: mcap $388B, debt $9B, op income $12B, free cash flow $9B
GM: mcap $49B, debt $110B, op income $10B, free cash flow $1B
Ford: mcap $48B, debt $139B, op income $11B, free cash flow $3B
Tesla makes more money than Ford or GM, while having much less debt. The fact that they sell fewer cars means their margins are higher, and they have more room to grow. Maybe they shouldn't be worth eight times more than Ford or GM, but they should definitely be worth more.
And if you believe that the industry is going electric, then Tesla is already there, while legacy auto has a lot of work to do, and Tesla's low debt load makes them better positioned to do lots of R&D.