Designers use font managers not only to help them organise the fonts per project, find duplicates etc. But also for performance by only activating the ones you need.
Even so, desktop font management tools seem like they've always been these third-party tools; you don't have closer integration in the desktop space of memory-saving features even close to what browser devs work on. Like more clearly managing browser tab (font) resources dynamically when not in use / not likely to be used anytime soon.