They don't call it when it's that close. They only call it when there is a winner that can't be overcome by the votes that haven't been counted yet. In the past most people went to the polls and voted. Absentee was meant for people that truly couldn't make it to the polls because of health or serious reasons.

Before COVID the absentee and military ballots usually wouldn't add up to enough votes to change an election given what they counted that night. That's why they were usually able to call the election that night. Then later in the week they would give the official totals.

According to the Pennsylvania Department of State “If you did not return your mail-in or absentee ballot and you want to vote in person, you have two options: Bring your ballot and the pre-addressed outer return envelope to your polling place to be voided. After you surrender your ballot and envelope and sign a declaration, you can then vote a regular ballot. If you don’t surrender your ballot and return envelope, you can only vote by provisional ballot at your polling place. Your county board of elections will then verify that you did not vote by mail before counting your provisional ballot.”

Also we'll know the results the evening of the election instead of waiting days for them to be counted.

Make sure to fill out your ballots properly, or better yet vote in person to be confident yours is counted.