1. The datasheet shows pin 16 as Rfb but does not explain its purpose. What does it do?
2. Also, there are two outputs on pins 1 & 2, is one of them to be always grounded as shown in Figure 3 and Figure 4? If so, then why even include it and not have just a single output?
3. Is it not possible to connect a voltmeter directly across pin 1 or pin 2 (and GND) to measure the output voltage of the ADC or must an op-amp to convert the current to voltage always be connected at the output?
1) RFb is an input for gain adjustment.
2) Just because two applications show a pin grounded doesn't mean it's "always grounded".
3) The DAC is current output, not voltage output; connecting a voltmeter across its output is meaningless.
4) I think you need to study up on multiplying DACs. First, read the The Principles of Operation section of the data sheet.
5) You can get a voltage-output DAC.
For the same reason there's not a power amp and a motor with a chicken on the end of the shaft built in to the design; because different applications have different requirements.