Why did the captive aliens in Independence Day II (Resurgence) still have their body armor?
The film's official novelisation offers a potential reason.
To help keep them alive (for future interrogation)
Although the aliens can adapt to Earth-like conditions, they apparently have an annoying tendency to die without warning. Keeping them in their exosuits may simply be a way of prolonging their lives.
This entire building had been designed to house the aliens and keep them alive so they could be studied. What little was known—or could be inferred—about their preferred habitat had been incorporated into the design. Not to keep them comfortable, but to extend their lives and therefore the opportunity to study them.
They seemed able to adapt to the terrestrial atmosphere and temperatures. As a result, some scientific personnel theorized that Earth-like conditions were necessary for advanced life forms to exist anywhere in the universe. Adams didn’t think the sample size was all that convincing.