star trek - Why do the Borg only assimilate at close range?

There is no in-Universe canon to support the Borg DID NOT have the ability to deliver nanoprobes to a species en masse.

We might make the assumption they did not since they were unable to assimilate or stop Species 8472 until they were aided by Voyager. But the help Voyager gave was not JUST the photon torpedoes modified with nanites loads. It was Voyagers stealth nanoprobes that the Borg lacked the creativity or research capacity to create.

Borg nanoprobes are highly infectious and very efficient at transforming an individual in a very short time, but the Borg would not assimilate entire species one specimen at a time. It would be inefficient and illogical.

In events with the Borg, we see them assimilating individuals, but the Borg only assimilate individuals, ships or small colonies for study and analysis at close range. It is more likely when they are assimilating a species, they use their Borg nanoprobes to infiltrate a natural resource required by the species and have the infected specimens return to a single collection point once they have been properly adapted.

While the Borg are likely to have the physical manpower to do overcome a species one planet and one individual at a time, it makes more sense to allow the potential species to infect themselves and then collect them afterward for processing. Their capacity for nanite production would basically allow them to seed entire worlds with nanites and return once their transformation was complete.

Nanites are perfect for concealing in food and water supplies due to their size. Only a species that knows what they are looking for, or perhaps has a technology designed to recognize inactive nanites would even know they are there.

Consider, the Borg only sent one ship to conquer the Federation after they decided to attack it. The confrontation at Wolf 359 indicated, one ship may have been enough.

Because we have not seen a multi-person deliver system does not mean they do not have one.

Once they have a specimen of the species they are going to absorb, they adapt the nanoprobes to make the transformation as efficient as possible. This does not take research, using strictly trial and error has worked for the Borg since very few species have the immunological response to resist the nanoprobes. To date, only Species 8472 have been biologically capable of resistance.

Their previous process of mass assimilation (whatever that process is) would not have worked against Species 8472 and they lacked the expertise to research why they were unsuccessful.

Voyager was successful in weaponizing the Borg nanoprobes because they were able to research why Species 8472 was resistant and using creativity make a process that would make the nanoproves invisible to Species 8472's hyper-efficient immune system. The weaponized versions of the Borg nanoprobes were redesigned by the Voyager crew to be ignored by the powerful immune system of Species 8472 until the process was unable to be stopped.

The innovation was not the torpedoes (though they certainly helped) it was the stealth nanoprobes that won the day. There is no reason to believe the Borg don't already possess a means of transforming a population, even if it is as prosaic as a flying vehicle dropping nanoprobes on a city. I would suspect they are more efficient than that. It's in their nature.