An ordinary analog card is built for a different voltage or signal range, with A/D converters of different precision and range, and different protective circuitry.
The 1734-IT2I has a sensitivity of 2.5 microvolts per count, while the 1734-IE2V has a sensitivity of 320 microvolts/count.
And ordinary analog modules don't have the onboard non-linear scaling lookup tables that are standard for thermocouples (in the case of the -IT2).
Another drawback to using sensitive input circuits is that they're generally delicate. Connect 24V DC to a thermocouple input module and you will damage it.
But the fact that I can connect a Cold-Junction Compensator to the module (another specific feature of the Thermocouple module) and get an accurate value of temperature in tenths of a degree is a big benefit to simplification of my programming and HMI.