As I have thought more about this, I think there actually may be a reason. When billable time is entered, an asset is debited (to the Billable time account). This asset is real, because time has been expended that can generate future revenue. To balance the asset, income is credited to Billable time - movement. But tax is not applied or due until invoicing. When it is, the liability in Tax payable is balanced in Accounts receivable. So if you applied tax at the moment of entering billable time, not only would you be recording a liability that does not technically exist yet, you would also have nowhere to post it without unbalancing the accounts.
Billable time entries are not just stored information. They are financial transactions. And only the value of the time has been accrued when the entry is made. The tax transaction comes later, when the invoice is raised. So implementing tax at the moment of billable time entry would require a departure from the way the program now operates. Whether that is feasible, I don’t know.