There isn’t any need to question why Amazon is worth a lot.

Investors thought - before AWS - that Amazon would win online retail (more than likely) and once they won, they would have pricing power to increase margins (doesn’t look likely).

With AWS, Amazon does have pricing power to an extent and it is a low marginal cost business. But, it could get Yahoo’d. Yahoo was doing well when it was taking money from venture backed unprofitable startup, but saw its fortunes rapidly turn when VC funding dried up. AWS won’t fall as fast or as hard but it will be affected.

Uber isn’t profitable - even their marginal profit is negative since they subsidize rides. No one knows how well Ubsr will do once it has to raise prices or cut drivers’ salary enough to be profitable.