Same reason regular companies cannot be banks in many countries , you would be able to give loans unfairly to your other business even if they are not credit worthy. It is wrong whether it is Best Buy or Amazon to be both marketplace and seller.
Even if the controls were somewhat weak in practice, his inability to commit to that even on paper is damning.
To be frank that bank policy sounds a lot like a pretense for protectionism to banks more than an antitrust measure so they don't have to worry about being disrupted by others winning big on bets they didn't take.
I recognize there may be very different values in play but I find calling that wrong bizzare and arbitrary.
Similar protections existed for ibanks not to do retail till a while before in the U.S.
It is to prevent a conflict of interest , the banking division can refuse to finance a competitor , give very cheap financing ignore the risk of the non finance business etc,
to put it in monopoly terms , you wouldn’t the player being banker to give cheap loans to himself while refusing to give you money right ?
Congress held this hearing as part of their looking at what changes to make to the law / changes to force the companies to make, so maybe they'll look at legislation that would change the situation (for example, some way of forcing Amazon to split "platform provider" and "product creator" into separate companies).
I've no idea if anything at all will come of it, or whether anything that does will be good or bad, but this hearing wasn't just about getting verbal commitments of being good from the CEOs (despite there being several questions did ask for verbal commitments in various areas).
Why is it impossible to commit? Restricting access to select people and firewall groups from these data , putting that in your contract with sellers are reasonable things he can do . It will not be perfect , people can still talk etc
Just because your safeguards will not be perfect doesn’t mean you shouldn’t have any. It is akin to saying as there are many zero days out there so I won’t secure my infrastructure at all .
Even if the controls were somewhat weak in practice, his inability to commit to that even on paper is damning.