This feature needs to be enabled by Zoom Support but currently will only be activated for selective use cases such as large-format broadcast events.
After contacting Zoom support, please allow up to 3 business days for the feature to be enabled.
Contact Zoom here
Full high-definition 1080p video is limited to Business and Enterprise plans.
A Zoom account is automatically considered a Business Plan when the account has 10 or more Pro paid plans.
In the image below, if HD has been enabled, you'll see 'always send 1080p video to attendees'. Check the box to enable.
Here's the full support article: