Not speaking for them, but the thought would be the custody of the asset is never transferred to a 3rd party like it is with the typical broker/client/clearing corporation arrangement that stocks use. If the asset is traded it goes straight from counterparty to counterparty. There's no need for an escrow arrangement because the execution is out of the hands of the counterparties.

A smart contract or wallet on the blockchain would hold a tokenized version of the security. In this way, folks could leave their tokens on a centralized brokerage if they chose to, or take custody of their shares by transferring the shares to a crypto wallet that they own the keys to.