Why can't I convert an estimate straight to an invoice?

There's no direct estimate-to-invoice button, because in field service the work happens between the two. You quote it, you do it, then you bill for what was actually done — which is rarely identical to the quote.

The flow is: approve the estimate → it converts to a job → the job gets done → create the invoice from the job, pre-filled with that job's line items.

This is what makes it possible to bill accurately when the scope changed on site, when a second visit was needed, or when a part came in at a different price.

Optional line items on an estimate are filtered out on conversion unless the customer selected them, so you don't inherit work nobody agreed to pay for.