why does my apple watch need to be near my iphone in order to send voice texts?

When this happens, have you checked to ensure that you are either still connected to the phone or that the Watch has connected to cellular? Switching from the paired phone to stand alone cellular can take a minute or two. Check the Control Center to make sure that it is connected. The cellular antenna icon will turn green, and just above it you will see green dots for the strength of the signal.

You might also wish to look at this support document that explains what you can do with the Watch when the paired phone is not nearby. How to use your Apple Watch without your iPhone nearby - Apple Support

There are 2 kinds of cellular service

1. Voice & SMS
2. Data

Cell phones (including the iPhone) contains BOTH kinds of radios.

The Apple Watch ONLY contains a "Data" radio. So if you want to send an SMS text message, the Apple Watch must relay the message to the iPhone so the iPhone's Voice & SMS radios can send the message.

The Apple Watch will cellular MUST use voice over internet protocols (VoIP) to do voice, but SMS is tied to the cellular voice radio and not available to Data only cellular services.

An Apple Watch can use Bluettooth, WiFi or cellular data to talk to the iPhone, but the iPhone must be accessible for that to happen.