I think the real reason, originally at least, was plausible deniability.

There was a ton of "look she's only the vice president, you can't blame her for Biden's unwavering support for genocide," early on.

At this point she's made it clear that Biden is governing exactly how she would anyways, so I don't know why they still keep up the charade, maybe inertia? Election is less than a month out anyways.