Why can't I modify token grants after I've created them?

If your token grants are published on-chain, you are not able to modify them. This is to create more confidence and assurance for stakeholders that they will receive tokens according to the agreed-upon schedule.

As the company admin, you still have the option to Archive a token grant, which means to freeze the token vesting schedule from progressing further and cancel all upcoming unvested token events. Any already vested tokens are claimable by the stakeholder.