The vast majority of satellites don’t look at space. That’s just space telescopes like Hubble, JWST, etc. And telescopes work pretty well right on the ground. Really just no connection between “discovering nebula” and “having thousands of satellites”.
Faint objects require a very long exposure time in the right wavelength. In the case of the nebula you’re talking about, O III emissions. The person who discovered it was specifically scanning the night sky with long O III narrowband exposures looking for unknown, faint sources of O III emission.
The combination of having to look in every direction, in every wavelength, with exposures of potentially unknown lengths, at many different scales means that it’s still possible to find things like this in the milky way that are undiscovered.
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