Why is the tax rate on my estimates zero?

Because nothing above it had a rate to give. A new estimate resolves its tax rate in one pass: the project's location first, then the workspace default, then zero. If your workspace default is blank and the project has no location rate, the estimate starts at 0%.

Set the workspace default at Settings, Workspace, Financials (/settings/workspace/financials). It applies to new estimates, invoices and orders across the whole workspace. See how to set your default tax rate, overhead and margin for the fields on that page.

If the project belongs to a location, that location's rate is used instead of the workspace default. A location with a blank rate inherits, so it falls back to the workspace value rather than to zero. That is worth checking when one project reads 0% and another reads correctly: how locations change your estimate defaults covers where that rate lives.

Two things follow from the rate being resolved at creation rather than looked up each time you open the document.

Changing the workspace default does not touch estimates that already exist. Every estimate already created keeps the rate it was given. Fixing the default fixes the next estimate, not yesterday's.

An estimate can be given its own rate in the estimate's settings, and that override survives everything above. Clearing it puts the estimate back on the resolved default, walking the same location, then workspace, then zero chain that creation walked, so a reset lands on exactly the rate a fresh estimate would have picked.

Invoices resolve one step earlier. An invoice built from an estimate carries that estimate's rate across, and only an invoice built from scratch falls through to the location and then the workspace default. So an invoice showing an old rate is usually showing you the estimate it came from.